Ukrainian soldiers sent to reclaim locations seized by pro-Russian militia in recent days, have been seen in the centre of Kramatorsk.
Kramatorsk’s airfield was recaptured on Tuesday after Ukraine’s acting President Oleksandr Turchynov announced an “anti-terror operation”.
Armored 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 Vladimir Putin warning Ukraine is “on the verge of civil war”.
Vladimir Putin made the comments in a phone call to German Chancellor Angela Merkel, after President Oleksandr Turchynov announced the start of the operation on Tuesday.
Russian officials said both leaders “emphasized 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 General Vasyl Krutov, speaking to reporters at Kramatorsk airfield late on Tuesday, emphasized 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.
Tensions 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.
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