Ukrainian government forces, backed by warplanes, kept up a military offensive to claw back lost territory from pro-Russian separatists on Tuesday while casting a nervous eye at Russian military exercises over the border.

Kiev's military said government forces had clashed 26 times with separatists in the Russian-speaking east in the 24 hours up to Tuesday morning, while fighter jets had struck at rebel positions and concentrations of military equipment. But it acknowledged that separatist forces had pushed it out of Yasynuvata, a railway junction near the main rebel-controlled city of Donetsk that it seized from separatists on Sunday.

Tension rose further with Ukraine denouncing Russian war games near the joint border as a "provocation" and alleging violations of Ukrainian air space by Russian warplanes and drones, as well as cross-border shelling from Russia. Defence officials said separatists had also opened fire on unarmed Ukrainian soldiers on Tuesday as they crossed back into Ukraine from Russia, where they had taken shelter from fighting.

Ukraine acknowledged on Monday that 311 soldiers and border guards had been forced by fighting with separatists to cross into Russia. It said they had destroyed their weapons before crossing the border, but the rebels said they had left them behind, enabling separatists to seize them.

A military spokesman said there had been no casualties from the attack, though he said three Ukrainian soldiers had been killed and 46 wounded in action against the separatists in the past 24 hours.