Six armoured troop carriers entered the eastern Ukrainian town of Slaviansk on Wednesday carrying the Russian national flag and the banner of pro-Russian separatists, a Reuters eyewitness said.
Several armed men, who wore different types of battle fatigues and appeared to be pro-Russia activists, sat atop each of the vehicles. The vehicles stopped outside the town's city hall which has for several days been occupied by separatists.
The second vehicle carried the emblem of the Donetsk People's Republic which a group of separatists proclaimed last week and which the Kiev authorities see as a first step to seeking to break from Ukraine and join with Russia.
The men on top of the troop carriers, some of whom were masked, were armed with Kalashnikov rifles, grenade launchers, knives and pistols.
The convoy came in from the direction of Kramatorsk, 15 kilometres (six miles) to the south, where Ukrainian airborne troops on Tuesday secured control over a military airfield.
The armed men waved to people as they drove into the town where separatists occupy several buildings including the police headquarters and the offices of the state security service.
Some people waved back and shouted "Well done lads !" People also chanted "Russia" Russia!"
There was no sign of Ukrainian troops in the city despite an announcement by Kiev authorities on Tuesday that a military-backed "anti-terrorist" operation was being extended to Slaviansk after the military action in Kramatorsk.
The origin of the troop carriers was not immediately clear.