(CNN) -- NATO's top military commander expressed concern Sunday about the buildup of Russian forces on Ukraine's border, as Moscow's forces consolidated their control of Crimea over the weekend. 
 
 Supreme Allied Commander Europe Gen. Philip Breedlove said Russia had a large force on Ukraine's eastern border and that he was worried it could pose a threat to Moldova's separatist Trans-Dniester region. 
 
 "The (Russian) force that is at the Ukrainian border now to the east is very, very sizable and very, very ready," Breedlove, a U.S. Air Force general, said. 
 
 "There is absolutely sufficient force postured on the eastern border of Ukraine to run to Trans-Dniester if the decision was made to do that, and that is very worrisome." 
 
 Russia said its forces complied with international agreements. Moscow annexed Crimea following a controversial snap referendum in the autonomous region last week that produced an overwhelming majority of votes in favor of leaving Ukraine to join Russia. 
 
 "Russia is acting much more like an adversary than a partner," Breedlove said, speaking at an event held in Brussels, Belgium, by the German Marshall Fund think tank. 
 
 Other officials expressed concern too. 
 
 "It's deeply concerning to see the Russian troop buildup along the border. It creates the potential for incidents, for instability," Tony Blinken, White House deputy national security adviser, told CNN's "State of the Union." 
 
 "It's likely that what they are trying to do is intimidate the Ukrainians. It's possible that they are preparing to move in." 
 
 Pro-Russia demonstrations planned 
 
 Pro-Russia demonstrations were planned for Sunday in Ukrainian cities outside Crimea, a Black Sea peninsula. The shows of favor for Moscow were scheduled for cities in Ukraine's south and east, where there are many ethnic Russians. 
 
 On Saturday, six Russian special forces armored personnel carriers broke through the gates of Belbek air base, firing warning shots into the air, a spokesman for Ukraine's Ministry of Defense in Crimea told CNN. One journalist was injured in the attack, said Vladislav Seleznev. 
 
 In a separate incident, pro-Russian self-defense forces stormed the Novofederoskoe military base, also in Crimea, taking control of it, a Ukraine Defense Ministry spokesman said. 
 
 Seleznev also said that Crimean self-defense forces and Russian special forces took a Ukrainian ship, the Slavutych. 
 
 The ship was captured "after a two-hour assault," he said, though the Ukrainian ship's crew members came ashore and didn't suffer any injuries. 
 
 Ukrainian forces on the Novofederoskoe base threw smoke bombs during the incident and retreated to the base's headquarters, Seleznev said in a Facebook posting. 
 
 The base's aviation brigade then sang the Ukrainian national anthem, lowered the Ukrainian flag and left the base, he said. 
 
 The White House urged Russia to open talks with the Ukrainian government. But in a statement, it also held the Russian military directly responsible for any casualties inflicted on Ukrainian military members -- whether from regular Russian troops or militias not wearing insignias. 
 
 Crimea remarries -- but that divorce will be messy 
 
 International monitors 
 
 The incidents occurred as international observers were set to arrive in Ukraine to monitor the security and human rights situation, in a posting expected to last at least six months. 
 
 But they will not enter the contested region of Crimea, a statement from the Russian Foreign Ministry said, because this "became part of Russia." 
 
 The Organization for Security and Cooperation in Europe will eventually deploy a total of 100 observers throughout Ukraine in hopes of "reducing tensions and fostering peace, stability and security." They will also check that the rights of ethnic minorities are being protected. 
 
 The OSCE may widen the mission to include 400 more monitors, and extend it for a second six-month period if requested by the Kiev government. 
 
 Russia, which is one of the OSCE's 57 member states, approved the mission, according to state-run Russian news agency RIA Novosti. 
 
 Russia's Foreign Ministry said it hoped the observers' work would stop nationalist extremists, who Moscow says have a strong influence on Kiev's government, and protect linguistic rights. Many Ukrainians speak Russian as their native language. 
 
 Russia has previously said it reserves the right to enter Ukrainian territory to protect ethnic Russians from what it says is a threat from ultranationalists and fascists. 
 
 Meanwhile, European leaders have demanded that independent monitors be granted access to the Crimean Peninsula. 
 
 OSCE monitors made multiple attempts to enter Crimea during the height of the crisis, when pro-Russian militias took control of the region. But armed men at the borders turned them back. 
 
 Russia sanctions: EU should leave trade alone 
 
 Claims on Crimea 
 
 Ukraine, the United States and other Western powers maintain Crimea is still a part of Ukraine. 
 
 "We must not allow a new division of Europe," German Foreign Minister Frank-Walter Steinmeier said Saturday in Kiev, where he met with with Ukrainian Prime Minister Arseniy Yatsenyuk and acting President Oleksandr Turchynov. 
 
 Russia insists its actions are legitimate. Crimea had belonged to Russia until 1954, when it was given to Ukraine. 
 
 The region also has a majority ethnic Russian population and other long historic ties to Russia. 
 
 Russia's Black Sea Fleet is based in the Crimean city of Sevastopol, making it strategically important to Moscow. 
 
 Moscow has doggedly pursued its own course, even as Western leaders have denounced its actions as violations of Ukraine's sovereignty and a breach of international law. 
 
 U.S. President Barack Obama plans to meet with leaders of the G7 group of industrialized nations this week to discuss Ukraine. 
 
 Russia has been excluded from the talks on the sidelines of a nuclear summit in the Netherlands. 
 
 A planned EU-Russia summit has also been canceled, as the West seeks to increase Moscow's isolation over its actions in Ukraine. 
 
 EU leaders imposed a new round of sanctions against 12 individuals last week, bringing the total number of people facing EU asset freezes and travel bans to 33. 
 
 The United States announced its own new round of sanctions against 20 individuals and a bank which U.S. officials say is linked to Putin and senior Russian officials. Washington had already announced sanctions on 11 individuals. 
 
 "What we've seen the President do in recent weeks, and what we'll see him do this week, is bring the world together in support of Ukraine, to isolate Russia for its actions ... and reassure our partners and allies in NATO and Europe," Blinken said. 
 
 Sanctions on Russia: Would World Cup boycott hit harder? 
 
 Opinion: Putin's breathtaking lies about Russia 
 
 Journalist Azad Safarov and CNN's Ivan Watson, Frederik Pleitgen, Laura Smith-Spark, Tom Watkins, Matt Smith and Ben Brumfield contributed to this report. ||||| Russia’s military staged a provocative new act of aggression Saturday, occupying a natural gas distribution center and village on a strip of Ukrainian land near the Crimean Peninsula and prompting Kiev’s Ministry of Foreign Affairs to denounce “a military invasion by Russia.” 
 
 The incident marked the first face-to-face standoff between the Ukrainian and Russian militaries outside the Crimean Peninsula, suggesting that Moscow is testing the will of Kiev amid fears of further Russian incursions in eastern and southern Ukraine. The move comes as the vote in Crimea begins on whether the residents there want to break away from Ukraine and join Russia. 
 
 On Sunday, the Ukrainian Defense Ministry said armed men raided a Ukrainian base under the control of Crimean self defense militias and siphoned gasoline stored there into oil tankers. 
 
 In the overnight incursion near Simferopol, about 25 men appeared with authorization from the pro-Russia Crimean government, according to Vladislav Seleznyov, a ministry spokesman. He said they filled two oil tankers that drove away, and two more arrived to be filled. 
 
 Russian troops in four helicopter gunships and three armored combat vehicles descended on the natural gas facility near the village of Strilkove about 1:30 p.m. local time, according to Ukrainian officials. The Russians said they had seized the site out of fears it would be targeted by “terrorists,” according to a Ukrainian Defense Ministry official who spoke on the condition of anonymity. 
 
 A Ukrainian border guard spokesman, Oleg Slobodyan, said Saturday evening that 120 Russian soldiers were still occupying the site and had not agreed to Ukrainian demands to leave. No shots had been fired, he said, but added that Ukrainian military forces had mobilized to just outside the city of Henichesk, putting themselves between the Russian force and the Ukrainian mainland. 
 
 In a statement, Ukraine’s Foreign Ministry demanded that the Russian side withdraw immediately and said that “Ukraine reserves the right to use all necessary measures to stop the military invasion by Russia.” 
 
 Ukraine’s Defense Ministry initially said in a statement that the Russians had left the area after a period of negotiation. But the ministry later corrected itself, saying its earlier statement was based on false information and that the Russians were still occupying the site. 
 
 It was not immediately clear whether the incursion was a temporary reconnaissance mission or a Russian attempt to secure the Ukrainian energy outpost just off the Crimean coast. 
 
 In Washington, President Obama’s top national security advisers gathered at the White House on Saturday afternoon to discuss the Ukraine crisis, and officials said Obama was being regularly updated on the situation. 
 
 Earlier in the day, Ukrainian officials reported a shootout between pro-Russian and pro-Ukrainian demonstrators that took place overnight in the eastern Ukrainian city of Kharkiv. 
 
 Two people were dead and several injured after a group of Russian separatists approached offices being used by pro-Ukrainian activists, according to Tatiana Gruzinskaya, spokeswoman for the city’s mayor. 
 
 The Russian Foreign Ministry issued a menacing statement Saturday in response to the violence. “There have been many appeals for Russia to protect peaceful civilians,” the statement said. “These appeals will be considered.” 
 
 That warning, which followed similar threats, is likely to add to fears that Russia will expand its intervention beyond Crimea and into eastern Ukraine. While Russia has blamed such clashes on right-wing Ukrainians and on the new government, Ukrainian officials placed responsibility squarely on Russia’s shoulders. 
 
 Speaking from the Ukrainian capital of Kiev, interim President Oleksandr Turchynov said that it was “Russian agents” who were “causing people to be murdered.” 
 
 As Turchynov spoke, tens of thousands of antiwar demonstrators turned out in Moscow to protest Russia’s intervention in Ukraine, marking the largest political rally in the Russian capital in more than a year. 
 
 ‘Stop interfering’ 
 
 Acting Interior Minister Arsen Avakov wrote on his Facebook page Saturday that it was not yet clear whether the people killed in Kharkiv were pro-Russian or pro-Ukrainian, and activists on both sides were arrested, officials said. 
 
 Ukrainian Foreign Minister Andriy Deshchytsya called on Russia to “stop interfering” in eastern Ukraine, saying that authorities here could handle issues of law and order in the region. He described the threatening statements from Russia’s Foreign Ministry as an attempt to “provoke” an escalation of the tense situation in the east. 
 
 [READ: Russia supporters in eastern Ukraine pose challenges to government] 
 
 Deshchytsya called for U.N. monitors to fan out across southern and eastern Ukraine to independently assess the situation on the ground. 
 
 He also reiterated Kiev’s position that Sunday’s referendum in Crimea would be “illegal” and would not be recognized by the international community. He emphasized the new government’s desire to avoid any military conflict and referred to the current stand off as more of a “diplomatic war.” But he added that he would travel to Brussels on Monday to meet with NATO’s secretary general to discuss “military and technical cooperation.” 
 
 Russia on Saturday blocked passage of a U.N. Security Council resolution that would have declared that Sunday’s referendum “can have no validity, and cannot form the basis for any alteration of the status of Crimea.” The veto was expected, but in a possible sign of Russian isolation, Moscow’s close ally China opted to abstain rather than join Moscow in vetoing the measure. The other 13 Security Council members supported the measure. 
 
 U.S. Secretary of State John F. Kerry on Friday warned against a “back-door annexation” of Crimea by Russia. But he conceded that six hours of talks in London with Russia’s top diplomat neither stopped the referendum nor opened a new diplomatic path for Moscow to step back from the Cold War-tinged standoff. 
 
 Tightening grip 
 
 The vote will be held under the eyes of Russian troops, who effectively took control of Crimea late last month after protesters overthrew the Ukrainian government. A Ukrainian Defense Ministry official said that a column of about 50 armored vehicles from the Russian Federation was observed Friday night moving from the city of Feodosia to Dzhankoi in northern Crimea. Ukrainian military spokesman Vladislav Seleznyov said that more Russian troops have moved in to occupy Ukrainian military bases that had been abandoned long before the crisis began. 
 
 Moscow’s tightening grip on Crimea and the gathering of Russian troops along the countries’ border have unnerved Ukrainians and left the country’s fledgling government concerned about further Russian military action. Kerry said the United States was “deeply concerned” about those deployments. 
 
 [READ: Ahead of Crimea vote, Ukraine officers express pro-Russia sentiments] 
 
 If Russia does move to annex Crimea after Sunday’s vote, it could trigger the harshest sanctions levied against Moscow by the United States and European powers since the Cold War. 
 
 The European Union is expected to impose travel bans and asset freezes Monday on Russians accused of complicity in Moscow’s military incursion and the intimidation of Crimea. The E.U. on Friday identified more than 120 individuals as potential sanctions targets. 
 
 The White House announced Friday that Vice President Biden will travel to Poland and Lithuania next week to discuss Ukraine and other issues with regional leaders. 
 
 Englund reported from Moscow. Pamela Constable in Simferopol, Carol Morello in Sevastopol, Kathy Lally in Moscow and Griff Witte in London contributed to this report. ||||| A number of seriously ill patients remain in the seized hospital in Simferopol, reports say 
 
 Armed men - said to be Russian troops and local militias - have seized a military hospital in Crimea, as Moscow tightens its grip on Ukraine's region. 
 
 The attackers marched into the hospital in the regional capital Simferopol, threatening staff and some 30 patients. 
 
 Pro-Russian troops are also blockading Ukrainian troops across Crimea. 
 
 The latest moves come ahead of Sunday's secession referendum in the autonomous region. Kiev and the Western nations describe the vote as illegal. 
 
 In other developments on Monday: 
 
 Russian Foreign Minister Sergei Lavrov says Moscow will send its "counter-proposals" to Washington to try to resolve the Ukraine crisis; the US earlier proposed to set up a contact group, renew direct Kiev-Moscow talks and also urged Russia to pull its troops in Crimea back to their bases 
 
 In a phone call, US President Barack Obama and his Chinese counterpart Xi Jinping urge Russia to respect Ukraine's territorial integrity 
 
 Russia's former tycoon Mikhail Khodorkovsky, who spent a decade behind bars, tells students in Kiev that Russia has severely violated international law by deploying troops in Crimea 
 
 'New directors' 
 
 Up to 30 men in military uniforms - some with truncheons - broke into the hospital, where Ukrainian soldiers and veterans were being treated. Some of the patients are reported to be seriously ill. 
 
 Armed men continue to blockade Ukrainian troops at their bases in Crimea 
 
 Crimea's pro-Russian authorities are urging local residents to "stop fascism" from Ukraine in the referendum - a claim firmly denied by Kiev 
 
 The hospital director said he was forced onto a bus and kept there for half-an-hour. 
 
 The attackers also herded staff into a reception to apparently meet "the new directors", Ukraine's Interfax-Ukraina news agency reports. 
 
 Separately, pro-Russian troops tried to capture a military transport base in Bahkchysarai, a town between Simferopol and Sevastopol. 
 
 The gunmen fired warning shots into the air, but Ukrainian soldiers repelled the attack. The negotiations are reportedly still continuing. 
 
 Crisis timeline 21 November 2013: President Viktor Yanukovych abandons deal on closer ties with EU in favour of closer co-operation with Russia 
 
 President Viktor Yanukovych abandons deal on closer ties with EU in favour of closer co-operation with Russia December 2013: Pro-EU protesters occupy Kiev city hall and Independence Square 
 
 Pro-EU protesters occupy Kiev city hall and Independence Square 20 February 2014: At least 88 people killed in 48 hours of bloodshed in Kiev 
 
 At least 88 people killed in 48 hours of bloodshed in Kiev 21 February: President Yanukovych signs compromise deal with opposition leaders 
 
 President Yanukovych signs compromise deal with opposition leaders 22 February: President Yanukovych flees Kiev. Parliament votes to remove him and sets elections for 25 May 
 
 President Yanukovych flees Kiev. Parliament votes to remove him and sets elections for 25 May 27-28 February: Pro-Russian gunmen seize key buildings in Crimean capital Simferopol 
 
 Pro-Russian gunmen seize key buildings in Crimean capital Simferopol 1 March: Russian parliament approves President Vladimir Putin's request to use Russian forces in Ukraine 
 
 Russian parliament approves President Vladimir Putin's request to use Russian forces in Ukraine 6 March: Crimea's parliament asks to join Russia and sets referendum for 16 March Law and order breakdown in Crimea Is Russian intervention legal? 
 
 Step-by-step, and meeting very little resistance, the pro-Russian troops are dismantling Ukraine's ability to resist in Crimea, reports the BBC's Christian Fraser, who is in the region. 
 
 Moscow has officially denied that its troops are taking part in the blockades, describing armed men with no insignia as Crimea's "self-defence" forces. 
 
 The government in Kiev - as well as the US and EU - accuse Russia of invading Ukraine, in violation of international law. 
 
 'Shameful silence' 
 
 Earlier on Monday, the Russian foreign ministry condemned "lawlessness" in eastern Ukraine, blaming far-right militants for "conniving" with the Kiev authorities. 
 
 In a statement, the ministry said the "well-equipped" gunmen opened fire on "peaceful protesters" in the eastern city of Kharkiv on 8 March. 
 
 The city has recently witnessed mass rival rallies, some of which were violent. 
 
 However, local Kharkiv police say they are treating the alleged shooting as a minor incident, according to Reuters. 
 
 The Russian statement also said the seven Russian journalists had been detained by police in Dnipropetrovsk, also in the east, who accused them of being interested only in "separate provocative stories". 
 
 "The Ukrainian authorities, in violation of all existing bilateral treaties, are not letting Russian citizens into the territory of Ukraine," the statement added. 
 
 And it also voiced Moscow's surprise over "the shameful silence of our Western partners, human rights groups and foreign media". 
 
 Ukraine has in the past firmly denied similar Russian allegations, instead accusing Moscow of distorting facts to justify its continuing military presence in Crimea. 
 
 Kiev points out that monitors of the Organization for Security and Co-operation in Europe have recently been prevented by pro-Russian militia groups from entering Crimea and a number of journalists have been beaten up by militias in the autonomous region. 
 
 Russian President Vladimir Putin insisted he has the right to protect Russian interests and the rights of ethnic Russians there. 
 
 On Sunday, he also defended the moves by Crimea's authorities to stage the referendum on 16 March. Mr Putin said "the steps taken by Crimea's legitimate authorities are based on international law". 
 
 However, German Chancellor Angela Merkel told him in a phone call that she considered the vote illegal. 
 
 Both EU leaders and the US have warned Moscow they would slap even tougher sanctions if Russian troops remained in Crimea. 
 
 Unrest in Ukraine erupted in November after former President Viktor Yanukovych's last-minute rejection of a landmark EU deal in favour of a bailout from Russia. 
 
 Mr Yanukovych was ousted last month, and a new government has been voted in by the Ukrainian parliament.
– Russia is hitting back at earlier reports that it had issued an ultimatum to Ukraine, with RT quoting a Defense Ministry spokesman as saying it's "total nonsense." "We’re interested in keeping friendly relations with the people of Ukraine and in preserving stability," another defense source says. Interfax first published the report—which said Russia would attack if all Ukrainian forces aren't out of the Crimea region by 5am tomorrow (meaning 10pm today EST)—citing sources in Ukraine's defense ministry, according to CNN. "If they won't surrender," the report said, "there will be a military storm on all UA (Ukraine Armed) military forces." The source said Russian troops were directly demanding surrender at various military bases in Crimea. A Ukraine Defense Ministry spokesperson also told the AP that Russian ships have trapped two Ukrainian warships in Sevastapol's harbor, and are demanding their surrender. Indeed, Ukraine's prime minister says that pro-Russian troops had surrounded or taken "virtually all" of the country's military facilities. In other developments: Russia is demanding that Ukraine reinstate a unity government negotiated on Feb. 21—which would mean re-installing Viktor Yanukovich as president. The idea appears to be a non-starter with the diplomats gathered in Brussels, however. Russia plans to press ahead with plans to build a bridge linking Russia with Crimea, Dmitry Medvedev said today, following a phone call discussing the crisis with Joe Biden, Sky News reports.