Kiev (CNN) -- Russia warned Tuesday that any use of force in Ukraine's eastern region could lead to civil war, as Kiev seeks to regain control after pro-Moscow uprisings in three cities. 
 
 Pro-Russian protesters seized government buildings in the cities of Donetsk, Luhansk and Kharkiv on Sunday. Rebels occupying Donetsk's regional government building Monday declared a "people's republic" and called for a referendum on secession from Ukraine to be held by May 11. 
 
 There were conflicting reports late Tuesday over whether demonstrators who seized control of a Security Service of Ukraine building in Luhansk took hostages. 
 
 An anti-terrorism unit outside the building claimed the pro-Russian demonstrators were holding hostages, Victoria Syumar, a security service spokeswoman, and Yarema Duh, spokesman for the National Security and Defense Council, told CNN. Fifty-one people were released from the building early Wednesday morning, the security service said in a statement. 
 
 But pro-Russian demonstrators holed up in the building denied they have taken anyone hostage, according to Reuters reports. 
 
 The security service also accused the demonstrators of placing explosive throughout the building, a claim the protesters denied, Reuters reported. 
 
 Russia's Foreign Ministry said reports that the protesters are facing a crackdown by Ukrainian authorities are of particular concern. 
 
 "We are calling for the immediate cessation of any military preparations, which could lead to civil war," it said in a statement on its website. 
 
 The ministry alleged that what it called "American experts from the private military organization Greystone," disguised as soldiers, as well as militants from the Ukrainian far-right group Right Sector, had joined Ukrainian forces preparing for the crackdown in the country's east. 
 
 Secretary of State John Kerry said Tuesday that Russian forces and special agents have been behind what he called the "chaos" in eastern Ukraine in the past 24 hours. Kerry described the developments as "more than deeply disturbing" and said they amounted to what could be a "contrived pretext for military intervention just as we saw in Crimea." 
 
 He followed up by warning of increased sanctions targeting Russia's banking, energy, mining and arms sectors if the Russians "cross over" into eastern Ukraine. Current sanctions target individuals over the Crimean annexation. 
 
 Kerry will meet Monday with his Russian, Ukrainian and European Union counterparts to discuss efforts to de-escalate the crisis, according to a statement released by the spokesperson for EU Foreign Minister Catherine Ashton. 
 
 Ashton "calls against any further destabilization of Ukraine, whether from the inside or the outside," the statement said. 
 
 Building cleared 
 
 Late Monday, special forces cleared armed protesters from the security service headquarters in Donetsk, acting President Oleksandr Turchynov's office announced. 
 
 But Ukrainian interim Deputy Prime Minister Vitaly Yarema was quoted by Russian state-run news agency ITAR-Tass as saying Tuesday that authorities are not going to storm the city's regional administration building. Yarema said the decision was made after talks with representatives of the protesters in the building. 
 
 Donetsk protesters dig in at government building 
 
 According to the deputy head of the National Security and Defense Council, Victoria Syumar, negotiations are under way between the protesters and police, coordinated by influential oligarch Rinat Ahmetov. 
 
 Donetsk is the hometown of ousted Ukrainian President Viktor Yanukovych, whose pro-Russian government was toppled in a popular revolt in February. 
 
 At a news conference in Kiev, interim presidential chief of staff Serhiy Pashinksiy said he had "convincing evidence" suggesting the protests were being financed by Yanukovych and his supporters. 
 
 In remarks posted by Russia's Interfax news agency and confirmed by his press secretary, Pashinksiy said authorities had frozen "dozens of bank accounts and seized colossal resources. All of these massive rallies are financed." 
 
 This could not immediately be independently verified. 
 
 'Anti-terrorist operation' 
 
 Police detained about 70 people in Kharkiv in an "anti-terrorist" operation that cleared the building in Kharkiv, an Interior Ministry spokeswoman said. 
 
 Special forces police from outside of the city carried out the operation, in which all those arrested were armed with knives or batons, spokeswoman Natalia Stativko said. 
 
 Those arrested have been taken to police detention centers in the cities of Poltava and Zaporijya and face charges relating to separatism, violence and taking part in mass protests, she said. 
 
 Earlier, in a Facebook message, Interior Minister Arsen Avakov said the city center had been closed to the public and urged people to remain calm. No guns were fired, he said. 
 
 Turchynov blamed "separatist groups coordinated by Russian special services" for the revolts, which he said echoed events leading to the Russian annexation of Crimea three weeks ago. 
 
 "Enemies of Ukraine are trying to play out the Crimean scenario, but we will not let this happen," he said in a televised message. 
 
 British Foreign Secretary William Hague said the protests bore "all the hallmarks of a Russian strategy to destabilize Ukraine" and warned of "grave economic and political consequences." 
 
 Meanwhile, the U.S. Navy warship USS Donald Cook is scheduled to enter the Black Sea no later than Thursday as part of the latest U.S. military effort to demonstrate support for Eastern European allies concerned about Russia's troop buildup, two U.S. military officials said. 
 
 The ship will conduct exercises and port visits while in the Black Sea, said the officials, who spoke on condition of anonymity because a formal announcement has not been made. 
 
 The move comes ahead of an April 15 meeting among top NATO political representatives, who will discuss and potentially approve NATO recommendations for additional military measures, including shortening the response time of NATO forces in a crisis. Other options include more military exercises with member nations, including the United States, and potentially providing military advisers to Ukraine. 
 
 Ukraine to Russia: I'm not your child 
 
 ‎Separatism outlawed 
 
 Turchynov reiterated Tuesday he would treat Russian separatists who have seized buildings in the east of the country as "terrorists" who will be prosecuted with the full force of the law. 
 
 His remarks to parliament came ahead of a vote in which it approved legislation outlawing groups and individuals who call for separatism. Of the 450 members of the Ukrainian parliament, 230 voted in favor of the bill. All members of the Communist Party abstained from voting. 
 
 Before the vote, a fight broke out in parliament when Petr Simonenko, the leader of the Communist Party, began to say lawmakers should listen to the demands of eastern Ukraine. 
 
 He defended demonstrators who have seized local government buildings, saying they are not doing anything different from what the current interim government has done. He also accused "nationalists" of starting the crisis in eastern Ukraine. 
 
 Simonenko was prevented from speaking further by a member of the far-right Svoboda party, and peace was restored in the parliament. 
 
 Russia urges talks 
 
 Russia, which has tens of thousands of troops near the Ukrainian border, said Monday that it was "watching closely" what was unfolding and told Ukraine to stop blaming it for Ukrainian problems. 
 
 President Vladimir Putin's government pushed Ukraine to set up a federal system in which regions with ethnic Russian majorities would have more autonomy, and its Foreign Ministry urged Ukraine to enter into talks over the issue. 
 
 "Ukrainian people want to get a clear answer from Kiev to all their questions. It's time to listen to these legal claims," a Foreign Ministry statement read. The Ukrainian government was acting "irresponsibly," it said. 
 
 At a news conference Tuesday, Foreign Minister Sergey Lavrov said Moscow has proposed involving Ukrainian presidential candidates in Russian, U.S. and EU talks on Ukraine. 
 
 He insisted that representatives of southeast Ukraine take part, too, according to the Interfax news agency. He said Russia was ready for the talks, which may take place in 10 days. 
 
 Moscow dispatched thousands of troops to Crimea in support of a pro-Russian movement that seized power in the semi-autonomous region shortly after Yanukovych fled to Russia. 
 
 The Black Sea peninsula was then subsumed into Russia after a referendum denounced as illegal by Kiev and the West. 
 
 Russia has called Yanukovych's removal a coup and says the interim government in Kiev is illegitimate. It has said it doesn't intend to invade eastern Ukraine but reserves the right to protect ethnic Russians there. 
 
 Ukraine says it retakes building seized by protesters 
 
 Is Russia done after Crimea? 
 
 FIRST ON CNN: Ukraine warns Russia in phone call: Don't invade 
 
 CNN's Laura Smith-Spark and Marie-Louise Gumuchian wrote in London and journalist Victoria Butenko reported from Kiev. CNN's Chelsea J. Carter, Barbara Starr, Kellie Morgan, Nick Paton Walsh, Boriana Milanova, Azad Safarov, Lindsay Isaac and Tom Cohen contributed to this report. ||||| LUHANSK, Ukraine (Reuters) - Pro-Russian separatists reinforced barricades around the state security building in the eastern Ukrainian city of Luhansk on Wednesday and called on President Vladimir Putin for help after the government warned it could use force to restore order. 
 
 Protesters were also engaged in talks to ease the standoff, which Kiev has said could provide a pretext for a Russian invasion, and lawmakers from eastern Ukraine proposed an amnesty for protesters to defuse tension. 
 
 The former KGB headquarters is one of three government buildings seized this week in eastern Ukraine by protesters demanding regional referendums on independence from Kiev. 
 
 Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine’s Moscow-backed president and the installation of a new pro-European government. 
 
 “Of course we must ask Russia to take us in because I don’t see an alternative,” said a man dressed in camouflage who gave his name as Vasiliy and said he was the commandant of the building. “Putin help us!” he said. 
 
 In a news conference held inside the occupied building late on Wednesday, Valery Bolikov, who said he was a representative of headquarters of the Southern and Eastern Army, said talks with authorities had failed to yet bring an agreement. 
 
 “Talks are continuing there are a few issues which are being dealt with but they haven’t come to their logical conclusion,” he said in Ukraine’s Security Services ornate conference hall. 
 
 While some protesters have championed the idea of joining Russia like Crimea, Bolikov said that their demands went no further than a referendum to vote whether to give Luhansk more autonomy as part of a federal structure in Ukraine. 
 
 “(We will leave the building) only after the fulfillment of our demands of the carrying out of a referendum on federalization,” he said. 
 
 Outside of the conference hall, masked men armed with kalashnikovs, pistols and guns lined the building’s many corridors. Tensions around the seizure of the building rose after the protesters broke into the security service’s arsenal. 
 
 One protesters put the arsenal at around 200-300 rifles. 
 
 Local police spokeswoman Tatyana Pogukai said that police were not taking any action while negotiations were ongoing. 
 
 “We don’t want any violence. No one needs any death or blood,” she said over the telephone. 
 
 FORCEFUL ANSWER’ 
 
 Protesters in Donetsk, to the south, remain in control of the main regional authority building, but authorities have ended the occupation in the city of Kharkiv. 
 
 “A resolution to this crisis will be found within the next 48 hours,” Interior Minister Arsen Avakov told reporters in the capital Kiev. 
 
 A pro-Russian protester stands at a barricade outside a regional government building in Donetsk, April 9, 2014. REUTERS/Maks Levin 
 
 “For those who want dialogue, we propose talks and a political solution. For the minority who want conflict they will get a forceful answer from the Ukrainian authorities,” he said. 
 
 Ukraine’s state security service said that 50 people had left the building in Luhansk overnight. Protesters confirmed that some had left. 
 
 “Those who left were not ready to stay and fight,” said Vasiliy, who said his “soldiers” would fight on until a referendum on independence from Kiev was held. 
 
 Occupation leaders said those who stayed behind were trained soldiers who had fought in Chechnya and Afghanistan. 
 
 “We won’t be the first ones to fire a shot but we will defend ourselves,” said Pavel Strupchevsky. 
 
 Protesters insist that they have no help from Russia and that no Russians are among their ranks, but Ukraine’s government says the actions are part of a Russian-led plan to dismember the country, a charge Moscow denies. 
 
 U.S. Secretary of State John Kerry accused Russian agents and special forces on Tuesday of stirring up separatist unrest and said Moscow could be trying to prepare for military action as it had in Crimea. 
 
 Russia denied the accusations on Wednesday and dismissed concerns over a troop buildup near the border with Ukraine in what has become the worst East-West crisis since the end of the Cold War in 1991. 
 
 “Russia has stated many times that it is not carrying out any unusual or unplanned activity on its territory near the border with Ukraine that would be of military significance,” the Russian Foreign Ministry said. 
 
 Slideshow (7 Images) 
 
 A lawmaker from the most popular political party in the east, the Party of Regions, on Wednesday said he planned to ask parliament to amnesty the protesters, following the success of a similar move to reduce tension in Kiev two months ago. 
 
 “The situation is so tense and complex that one stray word might cause a flare-up,” said Oleksandr Yefremov. “To prevent people suffering ... we are proposing a draft law on an amnesty.” ||||| Media playback is unsupported on your device Media caption The BBC's Steve Rosenberg: "The war of words between east and west continues" 
 
 The Ukraine authorities say they have seized back control of the regional administration building in the eastern city of Kharkiv from pro-Russia separatists. 
 
 They say they hope buildings in Luhansk and Donetsk will be freed shortly as well. 
 
 On Monday, pro-Russia demonstrators seized government buildings in the three cities. 
 
 Russia recently annexed the peninsula of Crimea following a referendum. 
 
 Kiev and the West say the referendum in the territory, where the majority of people are Russian speakers, was illegal. 
 
 Moscow has thousands of troops massed along its border with eastern Ukraine. 
 
 Although it insists it has no intention of invading Ukraine, it says it reserves the right to defend ethnic Russians in the country. 
 
 Ukraine Deputy Foreign Minister Danylo Lubkivsky has told the BBC Radio 4's Today programme that the situation in eastern Ukraine was "under control but remains dangerous". 
 
 Image copyright Reuters Image caption Interior ministry troops block a group of pro-Russia protesters in Kharkiv 
 
 Image copyright Reuters Image caption Pro-Russia protesters gather at a barricade outside the offices of the SBU state security service in Luhansk 
 
 Image caption Barricades can also be seen outside the Donetsk regional administration building 
 
 Image caption In the seized building in Donetsk, the man on the left says pro-Russia activists are "modern-day Oliver Cromwells", reports the BBC's Steve Rosenberg 
 
 Interim President Oleksandr Turchynov says several Ukrainian policemen have been injured in the operation to free the Kharkiv regional state administration. 
 
 Some 70 people were held without shots being fired, Ukraine's interior ministry said in a statement. 
 
 Media playback is unsupported on your device Media caption Lawmakers traded blows at Ukraine's parliament in Kiev 
 
 Mr Turchynov said those who seized the buildings would be treated as "terrorists and criminals" who would be prosecuted with the full force of the law. 
 
 Rebels occupying Donetsk's regional government building on Monday declared a "people's republic" and called for a referendum on secession from Ukraine to be held by 11 May. 
 
 There have been talks overnight in Donetsk between the authorities and pro-Russian activists who had occupied the regional administration building. 
 
 On Tuesday, Russia warned Ukraine to stop any military preparations, with the Russian foreign ministry saying in a statement that such preparations risked causing a civil war. 
 
 Russia is refusing to recognise the new authorities in Kiev who took power after pro-Moscow President Viktor Yanukovych was ousted in February. 
 
 'Whipping up tension' 
 
 Media playback is unsupported on your device Media caption Nato Secretary-General Anders Fogh Rasmussen: "I urge Russia to step back" 
 
 Nato's Secretary-General Anders Fogh Rasmussen has warned Russia that if it were to "intervene further in Ukraine it would be a historic mistake with grave consequences for our relationship with Russia and it would further isolate Russia internationally", he told the BBC. 
 
 Moscow denies orchestrating instability and separatist sentiment in Ukraine's industrial heartland. 
 
 In an article on the website of the UK's Guardian newspaper, Russian Foreign Minister Sergei Lavrov condemned what he described as the "groundless whipping up of tension" - he said the West was to blame for what was happening , because it had tried to force Ukraine to make a choice between east or west. 
 
 He also warned authorities in Kiev against any use of force against pro-Russian demonstrators. 
 
 On Tuesday, the Russian foreign ministry alleged that Ukraine was deploying "US mercenaries" from a private security firm dressed as Ukrainian special forces - but there has been no confirmation of that, says the BBC's Steven Rosenberg in Donetsk. 
 
 This is part of the continuing war of words between Russia and the West, our correspondent says. 
 
 But amid the war or words, there is still some talk of diplomacy, our correspondent adds: senior diplomats from the United States, Russia, Ukraine and the European Union are expected to meet within the next 10 days in an effort to defuse the tension. 
 
 The US and the EU have already imposed targeted sanctions on Russian and Ukrainian individuals over the annexation of Crimea. 
 
 BBC Moscow correspondent Daniel Sandford says that although events in Donetsk appear similar to those in Crimea, there are major differences. 
 
 Donetsk has many Ukrainian speakers as well as a Russian-speaking majority, and opinion polls there have shown considerable support for a united Ukraine, he says. 
 
 Eastern Ukraine was the political heartland of Mr Yanukovych and has a large Russian-speaking population. 
 
 Mr Yanukovych fled Kiev for Russia after months of street protests triggered by his refusal to sign an association agreement with the EU in favour of closer ties with Russia. More than 100 people died in the ensuing unrest.
– Pro-Russia protesters are bolstering their defenses in eastern Ukraine as Kiev threatens to use force to settle the situation. "A resolution to this crisis will be found within the next 48 hours," Ukraine's interior minister says, per Reuters. "For those who want dialogue, we propose talks and a political solution. For the minority who want conflict, they will get a forceful answer from the Ukrainian authorities." The separatist protesters, meanwhile, have built gasoline bombs and installed barricades of tires and crates around a seized security building in the eastern city of Luhansk. The separatists had been holding 60 people hostage, according to Ukrainian security services; they've now let 56 people leave, though it's not certain if those who have left were part of the reported 60, the AP notes. Protesters are also holding a building in the city of Donetsk; in Kharkiv, Ukrainian forces have retaken another site, they say, via the BBC. John Kerry says Russia is "the catalyst behind the chaos of the last 24 hours," and German leader Angela Merkel says it's "not clear that Russia is contributing to a de-escalation of the situation." For its part, Russia says the US and Ukraine "have no reason to be worried," Reuters notes. Despite a direct appeal from a separatist leader in Luhansk for Vladimir Putin to "have mercy on your fighters," Putin says Russia doesn't plan to invade, but says it has the right to stand up for its interests.