Armoured vehicles flying Russian flags have rolled into Slovyansk in eastern Ukraine.
Pro-Russian militia already control the city’s government and administrative buildings.
It is believed the convoy includes troops from a Ukranian airborne division who have switched allegiance.
One unnamed soldier involved in the column elaborated: “These Ukranian paratroopers have switched to the people’s side. You can see how many of us there are, the self defence army of Donbas.”
They stopped outside the town hall, which is occupied by separatists.
As the vehicles were driven in, some people waved and shouted: “Well done lads!” and “Russia” Russia!”
One soldier said he was a member of Ukraine’s 25th paratrooper division from Dnipropetrovsk.
“All the soldiers and the officers are here. We are all boys who won’t shoot our own people,” he said, adding that his men had had no food for four days until local residents fed them.
A spokesman for the separatists and a witness in Kramatorsk said the Ukrainian troops had given up their vehicles to the rebels after talks.
Overhead, a Ukrainian jet fighter carried out several minutes of aerobatics above the town’s main square in a show of strength by Kyiv’s forces.
A government official said the Ukrainian Defence Minister was travelling to Kramatorsk to try to clarify the situation.
Ukraine’s defence ministry denied that the men in uniform were Ukrainian, saying in a statement: “A column was blocked by a crowd of local people in Kramatorsk with members of a Russian diversionary-terrorist group among them. As a result of the blocking, extremists seized the equipment.”
The situation in the east of the country, the former heartland of ousted President Viktor Yanukovitch, is extremely tense.
Authorities in Kyiv and western allies claim tens of thousands of Russian troops are now massed along Ukraine’s border.
Many fear a Ukranian crackdown on pro-Moscow protesters will spark a full-scale Russian military intervention.