Armored vehicles bearing the Russian flag were seen driving in eastern Ukraine on April 16, as separatists stormed the city administration building in the main city of Donetsk.
Correspondents on the ground said the armored vehicles were seen in the city of Kramatorsk and in nearby Slovyansk.
Dozens of armed men in camouflage were seated atop the vehicles.
Video footage posted on the Internet appeared to suggest Ukrainian troops had defected to join pro-Russian separatists.
Ukraine's Defense Ministry confirmed pro-Russia activists had seized six armored vehicles in the town of Kramatorsk.
A Defense Ministry statement said "extremists" seized the vehicles and drove them to Slovyansk.
Meanwhile, separatists on April 16 broke into the city administration building in the region's main city of Donetsk.
A spokeswoman for the city council said at least 20 armed men took control of the building. An AFP correspondent says the protesters -- armed with Kalashnikov rifles -- entered the mayor's office, demanding that the region stage a referendum on turning Ukraine into a federation with broader local rights.
Separatists also continue to occupy official buildings in at least eight other eastern cities.
WATCH: Slovyansk Mayor Says Armed Separatists Hand Out Guns To Residents
Ukraine's Prime Minister Arseniy Yatsenyuk accused Russia of "exporting terrorism to Ukraine."
Yatsenyuk said that at planned four-party talks on April 17 in Geneva Ukraine will demand that Russia halt its alleged support for the separatists.
Ukraine's security forces on April 15 launched an "antiterrorist" operation against the separatists, deploying tanks outside Slovyansk and retaking a small military air base in Kramatorsk.
Ukraine's Security Service (SBU) said it had intercepted communications showing that Russian commanders in the east had issued "shoot-to-kill" orders to pro-Russian separatists after the launch of the security operation.
Russia denies accusations by Ukraine and the West it is behind the unrest, and has warned that the use of force against protesters could imperil the Geneva talks.
However, Russian Foreign Minister Sergei Lavrov said on April 17 that he still plans to take part in the Geneva talks.
Speaking in Brussels on April 16 after a meeting of the North Atlantic Council, NATO Secretary-General Anders Fogh Rasmussen said the alliance is planning military deployments by sea, air, and land within days to reinforce its presence on its eastern borders.
Rasmussen said the military steps being taken are those that "we think at this stage are necessary for our collective defense and deterrence." He said NATO hopes the Geneva talks will "pave the way for a peaceful and political solution to the crisis."