A combat vehicle with a Russian flag makes its way to the town of Kramatorsk, Ukraine, April 16, 2014. Evgeniy Maloletka/AP Photo

SLOVIANSK, Ukraine - Ukraine's "anti-terrorist operation" to take back areas in eastern Ukraine seized by pro-Russian forces faltered today as units of Ukraine's elite forces in tanks and armored personnel carriers defected to the pro-Russian side.

In at least two towns - Sloviansk and Kramatorsk - armored vehicles were seen flying Russian flags and carrying defected troops.

It comes after Ukraine made a big display of military might for journalists on Tuesday, showing off the hardware and forces that would take part in the operation. A little while later, helicopters containing special forces took off on their first mission, taking back the Kramatorsk airfield that had been occupied by armed gunmen.

As the operation continued today, Ukrainian jets and helicopters could be seen flying across the skies of the eastern Donetsk region.

Ukraine's acting president said Tuesday that the operation would be conducted "gradually with caution and responsibility."

"I want to emphasize that the aim of the operation is to protect the citizens of Ukraine, to stop terror, to stop criminals and to stop attempts at tearing our country apart," he said.

Pro-Russia protesters have been calling for greater autonomy for the eastern part of the country and for closer ties with Moscow.

Ukraine and the United States have repeatedly accused Russia of fueling the unrest here and today Ukraine's acting Prime Minister Arsenly Yatsenyuk said Russia is "exporting terrorism" to Ukraine.

"Russia must withdraw its sabotage groups, condemn terrorists and liberate all administrative buildings," Yatsenyuk said.