Ukrainian forces killed five militants during operations to take down pro-Russians' roadblocks in Slavyansk on Thursday, the Ukrainian
Interior Ministry said, in what appeared to be a significant escalation of violence in the country.
At the same time, Russia and the West continued their war of words over the handling of the crisis.
Russian President Vladimir Putin said the military operation being conducted by the Kiev authorities against protesters in eastern Ukraine was a "very serious crime" that would have consequences.
"If the Kiev regime has started to use the army against the population inside the country, it, beyond any doubt, is a very serious crime," Putin said at a media forum Thursday, according to Russian state TV channel Russia 24.
Conflicting accounts have emerged about the number of casualties in eastern Ukraine.
The government in Kiev confirmed operations to destroy three checkpoints around the city and said its forces killed five pro-Russian militants. A police officer was also injured, the Interior Ministry said.
But Stella Horosheva, a spokeswoman for the self-appointed pro-Russian mayor of Slavyansk, said an attack at an impromptu roadway checkpoint took the life of a pro-Russian militiaman and wounded another.
The Interior Ministry said leaflets had been distributed "which called on people to keep the peace, not leave their residences, to keep children inside, to not react to provocation and to not obey illegal orders issued by the self-proclaimed illegal authorities."
The government accused Slavyansk's self-appointed mayor, Vyacheslav Ponomaryov, of threatening to kill anyone possessing the leaflet.
Reports of threats against Slavyansk residents have not been independently confirmed by CNN.
In the Donetsk region, where pro-Russian protesters have tried to declare independence from Ukraine, gunmen opened fire on a Ukrainian military unit overnight.
One Ukrainian soldier was injured in the assault in the town of Artemivsk, Ukraine's Defense Ministry said Thursday.
But security forces fought off the attack and retained control of the facility, the ministry said.
Interior Minister Arsen Avakov on his Facebook page accused the group of roughly 70 attackers of trying to take weapons from the unit.