Ukrainian Prime Minister Arseniy Yatsenyuk has called on Russia to control its border to stop “terrorists” from crossing into his territory.
Mr Yatsenyuk said Kiev could solve the crisis quickly if Moscow stopped meddling in the situation.
But Russian Foreign Minister Sergei Lavrov warned that Kiev’s policies were pushing Ukraine into “fratricidal war”.
Earlier this week rebels and government forces were embroiled in some of the worst fighting of the crisis so far.
The separatists say they lost up to 100 fighters as they tried to seize Donetsk airport from pro-Kiev forces.
Ukraine’s interior ministry says the military is now in full control of the airport, although gunfire was reported in Donetsk itself on Wednesday.
Speaking on a visit to Germany, Mr Yatsenyuk said the situation in the east was deteriorating and Russia’s involvement was causing huge difficulties.
“A number of trucks full of live ammunition, full of Russian-trained guerrillas crossed the Russian border into Ukraine,” he said.
