Armored vehicles bearing the Russian flag have been seen driving in eastern Ukraine, Reuters reports.
The vehicles drove through the city of Kramatorsk, heading into the city of Slovyansk.
A witness told the Reuters news agency that the armored vehicles stopped outside the city hall in Slovyansk, which is still occupied by separatists.
Dozens of men in camouflage were seated on top of the vehicles.
Meanwhile, separatists on April 16 broke into the city hall in the region's main city of Donetsk.
Separatists also continue to occupy official buildings in at least eight other eastern cities.
Yesterday Ukraine's security forces launched an "antiterrorist" operation in the east of the country and retook a small military air base in Kramatorsk.
Russian President Vladimir Putin has warned that Ukraine is "on the verge of civil war."
Russia denies accusations by Ukraine and the West that it is organizing the unrest.