TORONTO (AP) — It began with Canada tweeting some not-so-friendly travel tips for the Russian military: a map showing Russia and labeling Ukraine as "not Russia." 
 
 This combination image shows a map tweeted by the Canadian Joint Delegation to NATO on Wednesday, Aug. 27, 2014, top, and a map tweeted in response on Thursday by the Twitter Feed of the Permanent Mission... (Associated Press) 
 
 That opening shot was enough to launch a Twitter war of words — and maps. 
 
 Canada's NATO Twitter account posted the map Wednesday with the explanation: "Geography can be tough. Here's a guide for Russian soldiers who keep getting lost and 'accidentally' enter Ukraine." 
 
 The tweet went viral, being retweeted more than 25,000 times by late Thursday, including by U.S. U.N. Ambassador Samantha Power. 
 
 Russia struck back by tweeting its own map showing Crimea as part of its territory. Russia annexed Crimea from Ukraine in March. 
 
 Kiev accused Russia on Thursday of sending tanks, artillery and troops across Ukraine's border. 
 
 Russia says its only active duty soldiers in Ukraine were the 10 captured earlier this week, who Moscow insists had mistakenly wandered across the border. 
 
 NATO estimates that as many as 1,000 Russian troops are fighting inside Ukraine with an additional 20,000 soldiers massing on the border for support or possible reinforcements. 
 
 "It's important to look at this exchange through the current context," said Rick Roth, the top spokesman for Canadian Foreign Affairs Minister John Baird. "The original tweet was in the context of Russian military personnel being found in sovereign Ukrainian territory, claiming they were lost. Nobody believes that." 
 
 Dutch Brig.-Gen. Nico Tak said there has been a significant escalation in both the level and sophistication of Russia's military interference in Ukraine. 
 
 "We have also detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred to separatist forces in eastern Ukraine," said Tak. 
 
 Baird called the recent events a "significant provocation" in advance of next week's NATO summit in Wales, where the alliance's leaders are expected to hold a special meeting with new Ukrainian President Petro Poroshenko. 
 
 Ukraine is not a member of NATO, so the alliance is not automatically obliged to come to its defense. 
 
 President Barack Obama ruled out the possibility that the U.S. will take military action over Ukraine and declined to characterize the assault as an invasion. ||||| Media playback is unsupported on your device Media caption Pro-Russian rebels enter the strategically-vital port of Novoazovsk on Wednesday 
 
 A pro-Russian rebel leader in eastern Ukraine has said 3-4,000 Russian citizens are fighting in their ranks. 
 
 Alexander Zakharchenko told Russian TV many of the Russians were former servicepeople or current personnel on leave. 
 
 He was speaking as rebels captured the town of Novoazovsk and threatened to take the city of Mariupol, after opening a new front in the south-east. 
 
 Reports say they are advancing on the city, a key port on the Sea of Azov. 
 
 Ukraine says Russian forces have crossed the border and are supporting the rebel attack, but Moscow has repeatedly denied arming or covertly supporting the rebels. 
 
 There are suspicions that Russia wants to divert Ukrainian forces from the besieged cities of Donetsk and Luhansk, further north. 
 
 Government forces have made significant advances against the separatists in recent weeks. 
 
 In other diplomatic developments: 
 
 An "increasing number of Russian troops" are involved in the fighting, the US ambassador to Ukraine, Geoffrey Pyatt, writes on Twitter 
 
 German Chancellor Angela Merkel demands an explanation from Russia's President Vladimir Putin amid the reports of an incursion 
 
 French President Francois Hollande says it would be "intolerable" if Russian troops are in Ukraine, and demands that Russia stop sending aid to the rebels 
 
 The Organisation for Security and Co-operation in Europe (OSCE) is holding a special meeting in Vienna to discuss developments in Ukraine. 
 
 Ukrainian forces 'leaving' 
 
 Mr Zakharchenko, who is prime minister of the self-declared Donetsk People's Republic, said Russian citizens were coming to Ukraine to fight because they felt it was their duty. 
 
 "There have been around 3,000-4,000 of them in our ranks," he said, adding that the rebels' struggle would have been much harder without them. 
 
 But he insisted that any Russians fighting on the rebel side were doing so voluntarily. 
 
 "Many former high-ranking military officers have volunteered to join us. They are fighting with us, considering that to be their duty," he said. 
 
 "There are also many in the current Russian military that prefer to spend their leave among us, brothers who are fighting for their freedom, rather than on a beach." 
 
 Ukraine's security and defence council confirmed reports that Novoazovsk had been captured by the rebels, whom they described as "Russian troops". 
 
 It said it had withdrawn its forces to save their lives, and they were now reinforcing Mariupol's defences. 
 
 Image copyright AP Image caption There has been shelling in the town of Novoazovsk for several days 
 
 Image copyright AP Image caption Ukrainian forces are being deployed to help defend the southern city of Mariupol 
 
 Image copyright AFP Image caption Shelling is continuing in the city of Donetsk hitting buildings including this school 
 
 Image copyright Reuters Image caption Railway lines in eastern Ukraine have been destroyed in the fighting 
 
 A Ukrainian company commander, Vladimir Shilov, told Ukrainian TV that he had heard from sources inside the town that it was blocked by tanks and no-one was allowed to leave. Local officials had already fled to Mariupol, he added. 
 
 A spokesman for the rebels told Interfax news agency that Novoazovsk was under their control and they would soon "liberate" Mariupol. 
 
 Reports on Twitter suggested the rebels were already advancing towards Mariupol, and rebels said they had captured checkpoints to the west of the city. 
 
 The port has until now been peaceful and cut off from rebel positions. 
 
 Rebels have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled. 
 
 Analysts say the separatists could also be seeking a land link between Russia and Crimea, which also would give them control over the entire Sea of Azov. 
 
 Russia annexed the Black Sea peninsula of Crimea from Ukraine in March. 
 
 War in eastern Ukraine: The human cost 
 
 At least 2,119 people had been killed and 5,043 wounded since mid-April, a UN report on 7 August said 
 
 951 civilians have been killed in Donetsk region alone, the official regional authorities said on 20 August 
 
 Official casualty counts only record certified deaths while in some particularly dangerous parts of the war zone, such as Luhansk region, victims are said to have been buried informally, for instance in gardens 
 
 Rebels (and some military sources) accuse the government of concealing the true numbers of soldiers killed 
 
 155,800 people have fled elsewhere in Ukraine while at least 188,000 have gone to Russia 
 
 Ukrainian President Petro Poroshenko promised a roadmap for peace in the east on Tuesday after holding his first direct talks on the crisis since June with Mr Putin. 
 
 Mr Putin said Russia would assist any ceasefire talks, but that stopping the fighting was a matter for Ukraine alone. 
 
 Are you in south-east Ukraine? Have you been affected by recent events in the area? You can send us your experiences by emailing haveyoursay@bbc.co.uk. ||||| Image copyright Reuters Image caption Fighting has been raging in the east between the Ukrainian military and pro-Russian rebels 
 
 Ukrainian President Petro Poroshenko has told President Vladimir Putin that reports of Russian tanks moving into Ukrainian territory are unacceptable. 
 
 Ukraine's interior minister said earlier that three tanks had entered rebel areas of the east, and fighting was under way. 
 
 Arsen Avakov said that the tanks entered Ukraine along with other armour through a border checkpoint controlled by rebels in the Luhansk region. 
 
 Russia denied the report. 
 
 The foreign ministry said it was "another fake piece of information". 
 
 Meanwhile, a vehicle belonging to a separatist leader was blown up in the eastern city of Donetsk late on Thursday. 
 
 The BBC's Steve Rosenberg says the van belonged to Denis Pushilin, the self-declared leader of the Donetsk People's Republic. 
 
 Mr Pushilin was not inside but pro-Russian separatists, who control the city, said four people were injured. 
 
 It is not clear what caused the explosion. 
 
 Image copyright AP Image caption A vehicle belonging to a separatist leader was destroyed in an explosion in Donetsk on Thursday 
 
 Image copyright AFP Image caption Denis Pushilin, the self-declared leader of the Donetsk People's Republic, was not in the vehicle at the time 
 
 Earlier, Moscow accused President Poroshenko of not delivering on his pledge to stop fighting in the east. It wants an investigation into allegations that Ukrainian forces used banned incendiary bombs against civilians. 
 
 "The lack of any progress whatsoever in efforts to stop the violence and halt military operations... is causing increasing concern," Russian Foreign Minister Sergei Lavrov said. 
 
 He said that reports in the Russian media that incendiary bombs were being used was another "cause for special concern". 
 
 The bombs are designed to trigger major firestorms and were often used during the Vietnam War before they were banned by the United Nations. 
 
 Ukraine has denied using the weapons. It has accused Russia of allowing the rebels to deploy three Soviet-era T-72 tanks across the border into the east of the country. 
 
 Ukraine's presidency said in a statement that Mr Poroshenko, who was elected last month, had a "substantial" conversation by telephone with Mr Putin on Kiev's plan of bringing peace to the east. 
 
 'Destroyed' 
 
 Mr Avakov said the tanks had crossed the border from Russia along with armoured personnel carriers and artillery pieces in the Dyakove area of Luhansk region, before moving into the neighbouring Donetsk region. 
 
 Two headed towards the town of Horlivka and were attacked by government forces, he added. 
 
 Image copyright Reuters Image caption Rebel fighters could be seen in Snizhne on Thursday 
 
 Unverified video has been posted on YouTube of a battle tank rolling down a street said to be in Snizhne. The footage was shot from a flat overlooking the street. 
 
 A second clip, also said to have been recorded in Snizhne, shows a similar tank passing a roundabout. 
 
 There was no confirmation about the tanks from the insurgents. 
 
 Pro-Russian separatists in the Donetsk and Luhansk regions declared independence after holding referendums last month which were deemed illegal by the government in Kiev. 
 
 Ukraine and the West accuse Russia of supporting and arming the rebels - a claim denied by Moscow. 
 
 The rebellion began amid the turmoil which followed February's ousting of President Viktor Yanukovych, whose last-minute decision not to sign a landmark treaty with the European Union in November sparked mass street protests in Kiev.
– Ukraine's Ministry of Foreign Affairs tweeted a pair of hashtags last night—#UkraineUnderAttack and #RussiaInvadedUkraine—which appear to sum up the situation on the ground, the Washington Post reports. Pro-Russia separatists backed by Russian troops, artillery, and armored vehicles are now attacking on two fronts, authorities in Kiev say, with fierce fighting along a new southeastern front. The Russian tanks and armored personnel carriers seen in the area—which would give Russia a land bridge to Crimea—appear to have come straight over the border instead of from separatist-controlled areas in eastern Ukraine, the Los Angeles Times reports. President Petro Poroshenko canceled a trip to Turkey, citing sharp aggravation of the situation in Donetsk region, as Russian troops were actually brought into Ukraine," according to a statement. Russia has not acknowledged any troop movements. A rebel commander says there are around 4,000 Russians fighting with the separatists, but they are all volunteers. "Many former high-ranking military officers have volunteered to join us. They are fighting with us, considering that to be their duty," he tells the BBC. "There are also many in the current Russian military that prefer to spend their leave among us." The US has expressed its "deep concern" about Russian involvement in the rebel offensive. "Russia is clearly trying to put its finger on the scale to tip things back in favor of its proxies," a senior American official tells the New York Times. "Artillery barrages and other Russian military actions have taken their toll on the Ukrainian military." After days of fighting, the strategic southeastern town of Novoazovsk now appears to be under rebel control, the AP reports.