Ukrainian troops sent to reclaim locations seized by pro-Russian militia in recent days, have been seen in the centre of Kramatorsk.The town's airfield was recaptured on Tuesday after the acting president announced an "anti-terror operation".Armoured personnel carriers in the town were blocked by pro-Russian locals who were talking to the soldiers.Tension is running high, with Russia's President Putin warning Ukraine is "on the verge of civil war".Mr Putin made the comments in a phone call to German Chancellor Angela Merkel, after acting President Olexander Turchynov announced the start of the operation on Tuesday.Russian officials said both leaders "emphasised the importance" during the conversation of planned four-way talks on Thursday between senior diplomats from Russia, the EU, the US and Ukraine.However, Ukrainian and Western officials have accused Russia of being behind the pro-Russia activism in the region. Moscow strongly denies fomenting the unrest.Ukrainian counter-terrorism chief Gen Vasyl Krutov, speaking to reporters at Kramatorsk airfield late on Tuesday, emphasised that armed individuals had crossed the border into Ukraine."We have to deal with a very serious, highly skilled and very professional opponent," he said according to Interfax-Ukraine news agency."They display a very high level of preparedness, tactical and practical skill. These people must have been to hot spots across the world and on their own territory."Last month's annexation of Crimea by Moscow was preceded by the presence of uniformed, armed men believed to be Russian troops, although in greater numbers than the similarly dressed and equipped men who have been seen in eastern Ukraine, correspondents say.The Kremlin has condemned the Ukrainian military operation in eastern Ukraine as an "anti-constitutional course to use force against peaceful protest actions".Russia's foreign ministry has expressed "deep concern" at reports of casualties in eastern Ukraine, but these cannot be confirmed.There have been unconfirmed reports of people being wounded in parts of Kramatorsk, but BBC correspondents in the city have so far not seen any violent confrontations since the airfield was retaken yesterday, when warning shots were fired.Buildings seizedTensions have mounted in recent days after pro-Russian rebels seized buildings in about 10 towns and cities across eastern Ukraine.They are demanding greater autonomy or referendums on secession from Ukraine.UK Foreign Secretary William Hague said Russia had sent in "thinly disguised" armed groups to spearhead the occupation of buildings.Speaking in London, he said Moscow faced "serious long-term consequences" if it continued to destabilise Ukraine.Mr Turchynov said on Tuesday that troops had regained control of a military airfield in Kramatorsk after fighting with armed pro-Russian groups.He said the aim of the operation in the east was to "protect Ukrainian citizens, to stop the terror, to stop the crime, to stop the attempts to tear our country apart".Overnight, a crowd of up to 100 people were reported to be outside the airbase with petrol bombs lined up on the ground.Elsewhere, Ukrainian armoured vehicles have been assembling north of the nearby town of Sloviansk."They [separatists] must be warned that if they do not lay down their arms, they will be destroyed," Ukrainian Security Service General Vasyl Krutov told reporters.Russia took control of the Ukrainian region of Crimea last month, after it held a controversial referendum on self-determination.The US has said it is "seriously considering" adding to sanctions imposed after the annexation of Crimea.The White House has described Ukraine's military operation as a "measured" response to a lawless insurgency.(BBC)ANN.Az