Pro-Russian militants in eastern Ukraine have seized six Ukrainian armoured vehicles, the defence 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, armoured vehicles appeared in the centre of the town early on Wednesday.
BBC journalists witnessed civilians, at least some of whom appeared to be local people, challenging soldiers, who were also blocked by a crowd a few kilometres outside the town.
One officer said he had not "come to fight" and would never obey orders to shoot his "own people".
"A column was blocked by a crowd of local people in Kramatorsk with members of a Russian diversionary-terrorist group among them," the defence 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.
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.
Ukraine's "anti-terrorist" operation is looking more and more a non-event - or worse, an outright fiasco, reports the BBC's David Stern in Donetsk.
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 Defence Minister Mykhailo Koval headed for the east of the country to monitor the progress of the "anti-terrorist operation" announced by acting President Olexander Turchynov on Tuesday.
In the city of Donetsk, where activists have been occupying the regional government building since 6 April, 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' defences.
In Brussels, Nato Secretary General Anders Fogh Rasmussen promised "more planes in the air, mores ships on the water, more readiness on the land".
He called on Russia to make clear it did not "support the violent actions of well-armed militias or pro-Russian separatists in eastern Ukraine".
The Nato alliance includes two ex-Soviet Baltic republics with large ethnic Russian communities, Latvia and Estonia, while other members such as Poland share borders with Russia.
Four-way talks are due to take place on Thursday in Geneva between diplomats from Russia, the EU, the US and Ukraine.
Ahead of the annexation of Crimea, masked soldiers believed to be Russian troops appeared at strategic points across the peninsula alongside "self-defence" units, said to have been formed locally.
Speaking in London, UK Foreign Secretary William Hague said Russia had sent "thinly disguised" armed groups into eastern Ukraine to spearhead the occupation of buildings.
Ukraine's Security Service (SBU) has meanwhile published what it says is a batch of intercepted conversations between the Russian secret services and pro-Russian forces in eastern Ukraine.
Russian security service officers are heard to order forces in eastern Ukraine to "shoot to kill" when dealing with Ukrainian troops who do not surrender, according to the transcripts.
Counter-intelligence spokesman Vitaliy Naida told reporters the same Russian agents had been involved in the run-up to Russia's Crimea annexation.
The intercepts could not be independently verified. Moscow maintains the pro-Russian protests in eastern Ukraine are the result of grassroots activism.
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