Ukrainian forces have killed up to five pro-Russian militants as they fought to regain control of the eastern city of Sloviansk, a separatist stronghold.
Kiev's troops clashed with armed separatists at three checkpoints set up by the insurgents to block access to the city, Ukraine's interior ministry said.
"During the clashes five terrorists were killed," a spokesman said, adding that a soldier was also wounded.
Russian president Vladimir Putin condemned the military operation as a criminal act.
"If the regime in Kiev has begun using the army against the population inside the country, then this is undoubtedly a very serious crime," Putin said.
Photos from the scene showed Ukrainian security forces backed by armoured vehicles moving past a road block that was reportedly set on fire and left by separatists to slow down the army's advance.
Army helicopters were also seen hovering over Sloviansk's suburbs in videos uploaded online.
The operation came as Kiev resumed military operations to oust pro-Russian militants who have been occupying government buildings in numerous eastern cities.
Five people were injured in the successful night raid to regain the port city of Mariupol.
The "anti-terrorism" operation was announced by Ukraine's acting president, Oleksandr Turchynov, two days ago.
Earlier this week, Kiev and the US state department published photos supporting claims that Russian Special Forces were actively leading or supporting eastern Ukraine insurgents.
Russia Foreign minister Sergei Lavrov had warned that Moscow would retaliate, possibly launching a vast scale military operation as it did in Georgia's breakaway region of South Ossetia in 2008, if Russian interests were attacked.
Lavrov said: "If we are attacked, we would certainly respond. If our interests, our legitimate interests the interests of Russians have been attacked directly, like they were in South Ossetia for example, I do not see any other way but to respond in according with international law."