KIEV -- Ukraine's acting president, Oleksandr Turchynov, said Monday that Kiev considers actions of pro-Russia activists in the Donbass region in eastern Ukraine as Moscow's "aggression."

"What is going on in Donbass is not confrontation between Ukrainians. It is hidden and sometimes not hidden aggression of Russia against our country," Turchynov said during a conciliatory council here.

Describing Russia's reaction to Kiev's "counter-terror operation" in eastern Ukraine as "inadequate," Turchynov accused Kremlin of offering "public support for terrorist offence."

Turchynov ordered a "counter-terror operation" in eastern Ukraine after pro-Moscow activists seized several government buildings there, demanding closer ties with Russia.

The Russian Foreign Ministry warned that talks due to be held on Thursday between Ukraine, Russia, the United States and the European Union might be aborted if Kiev uses force to quell the protests.

Tensions in eastern Ukraine escalated on Sunday following the armed confrontation between Ukrainian security forces and pro-Moscow protesters.

Russia claims that a security officer and a pro-Russia activist were killed in a raid launched by Ukrainian special forces in a town of Slavyansk.

Meanwhile, Ukrainian media reported that pro-Moscow activists have killed two Ukrainian security officers and two civilians during the violence.