Six Ukrainian armored vehicles have been seized by pro-Russian militants in eastern town of Kramatorsk, the defense ministry in Kiev says.
Reports say the occupants were disarmed after the vehicles were blockaded by locals in the city of Kramatorsk.
The incident comes a day after the military began an operation to remove pro-Russian protesters from public buildings across eastern Ukraine.
Meanwhile, NATO is increasing activity in member states bordering Russia.
After Ukrainian forces recaptured an airfield outside Kramatorsk on Tuesday, armored vehicles appeared in the centre of the town early on Wednesday.
“A column was blocked by a crowd of local people in Kramatorsk with members of a Russian diversionary-terrorist group among them,” the defense ministry said its statement.
The military vehicles were then taken to Sloviansk where they are being held by “people in uniforms who have no relation to Ukraine’s armed forces,” the ministry said.
The Ukrainian troops appear to have been disarmed before being fed by pro-Russian militants at a cafe in Sloviansk and then put on a bus back to their home city of Dnipropetrovsk.
Six Ukrainian armored vehicles have been seized by pro-Russian militants in eastern town of Kramatorsk
In another incident, several hundred residents of Pchyolkino, south of Sloviansk, surrounded another column of 14 Ukrainian military vehicles.
After the crowd was reinforced by pro-Russian gunmen, negotiations ensued and the troops were allowed to drive their vehicles away, but only after agreeing to surrender the magazines from their assault rifles.
The episodes come amid increasing tension across eastern Ukraine, blamed by the Kiev government and the West on covert Russian intervention in the region – an allegation denied by Moscow.
The crisis escalated this month after pro-Russian rebels occupied buildings in about 10 towns and cities, demanding greater autonomy or referendums on secession.
Tens of thousands of Russian soldiers are believed to have massed on Ukraine’s borders since Russia took control of the Ukrainian region of Crimea last month, following a controversial referendum on self-determination.
As tensions rose, Ukraine’s acting Defense Minister Mykhailo Koval headed for the east of the country to monitor the progress of the “anti-terrorist operation” announced by acting President Oleksandr Turchynov on Tuesday.
In the city of Donetsk, where activists have been occupying the regional government building since April 6, pro-Russian gunmen have taken control of the mayor’s office.
They told an AFP correspondent their only demand was for the region to stage a referendum on turning Ukraine into a federation with broader local rights.
Meanwhile NATO announced it was beefing up its eastern members’ defenses.
In Brussels, NATO Secretary General Anders Fogh Rasmussen promised “more planes in the air, mores ships on the water, more readiness on the land”.
Anders Fogh Rasmussen called on Russia to make clear it did not “support the violent actions of well-armed militias or pro-Russian separatists in eastern Ukraine”.
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.
Ukrainian soldiers sent to reclaim locations seized by pro-Russian militia in recent days, have been seen in the centre of Kramatorsk
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.
A gun battle has erupted in the eastern Ukrainian town of Kramatorsk, Ukraine’s acting Interior Minister Arsen Avakov says.
Arsen Avakov said it began when unidentified gunmen tried to storm local administration buildings and police fired back.
Several other official buildings were reported to have been seized in eastern Ukraine on Saturday.
The confrontations come amid rising tension between the new government and pro-Russia protesters.
Earlier, gunmen occupied a police station and a security services building in the town of Sloviansk. Official buildings in Druzhkovka were also reported to have been taken over.
Several official buildings were reported to have been seized in eastern Ukraine (photo AFP)
A Donetsk regional police chief also quit after pro-Russia crowds marched on a police station demanding his resignation.
Eastern Ukraine has a large Russian-speaking population and has seen a series of protests since the ousting of pro-Russian President Viktor Yanukovych in February.
The new government in Kiev accuses Moscow of orchestrating the unrest in eastern Ukraine. But Russia denies responsibility.
Protesters in largely Russian-speaking Donetsk, 80 miles from Sloviansk, have been occupying government buildings for days and demanding a referendum on becoming part of Russia.
Trouble continued in several towns and cities on Saturday despite the Kiev government setting a deadline of Friday for all occupations to end.
“Kramatorsk. An attack is under way. Unknown persons fired shots at the district police department. The police are firing shots in response. A shootout is under way,” Arsen Avakov wrote on his Facebook page late on Saturday.
He also reported that an attack on police buildings in Krasny Liman late in the day had been repelled. The gunmen there had been equipped with Russian-made Kalashnikov assault rifles, he added.
There were no reports of casualties in Kramatorsk or Krasny Liman and Arsen Avakov’s comments could not be independently verified.
Earlier, in the town of Sloviansk, dozens of unidentified armed men in camouflage uniform seized the police station and security service premises.