An unspecified number of pro-Russian separatists have been killed, injured and arrested in a Ukrainian government operation in the eastern city of Slovyansk, interim president Oleksandr Turchynov has said.
Russia has called for an emergency meeting of the UN Security Council after stating that the use of the military against the Ukrainian people was "leading Ukraine to catastrophe".
Turchynov said that the assault on rebels was "greatly complicated" by the fact that the unrest is taking place in heavily-populated cities.
Two Ukrainian army service personnel were killed in the fight for Slovyansk, Turchynov said, accusing pro-Russian forces of hiding behind civilians and hostages.
All pro-Russian checkpoints around the city had been captured, Turchynov added, though separatists have claimed several are still under their control.
The pro-Russian militants earlier shot down two Ukrainian attack helicopters, killing two army personnel, a pilot and a serviceman.
In reaction to the latest offensive, Russian president Vladimir Putin's spokesperson said that the Ukrainian government's actions ended any hope of keeping the Geneva peace accord alive.
"It looks as though at the same time Russia is employing efforts on the deescalating and settling of the conflict, the Kiev regime has moved on to firing from military aircraft at civilians in populated areas" Russian press secretary Dmitry Peskov said.
"The beginning of this retaliatory operation has practically destroyed the last hope on the Geneva Accords.
"During a recent visit to Minsk, Putin called such a possible operation criminal. Regrettably, the ongoing events have fully confirmed this assessment" Peskov concluded.
Turchynov added that all pro-Russian checkpoints in the eastern city had been captured by the Ukrainian military.