KIEV, Aug. 5 - 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, Reuters reported.

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 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.

Defense 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.

Defense spokesman Andriy Lysenko said Ukrainian forces, apart from engaging separatists, had come under mortar and artillery attack from Russia and there had been violations of Ukrainian airspace by Russian planes.

He expressed alarm at Russian military exercises this week near their long joint border, including deployment of 100 fighter aircraft.

"Ukraine regards the carrying out of such unprecedented military exercises on the border with Ukraine as a provocation," he said, while a foreign ministry statement called for Russia to pull its forces back.

Ukrainian security officials said separatist fighters were launching counter-attacks to break a tightening noose around the rebels who seek to set up pro-Russian 'people's republics' in the east of Ukraine.

Lysenko conceded that Ukrainian troops had abandoned control of Yasynuvata, a railway hub whose recapture from the separatists had been trumpeted.

Rebels at the scene on Tuesday told Reuters said they had lost at least one fighter, with three others injured, to shelling by Ukrainian forces.

"Every 15 minutes a shell comes over. They (the Ukrainians) fire from a distance and avoid fighting us directly," one separatist fighter said.

Lysenko told reporters that Ukraine had moved up its main troops near Donetsk and Luhansk, but he would not be drawn on when they would launch an operation to storm the towns.

"We'll not talk about starting an offensive. We will speak only about liberating these towns," he said. (rt/ez)