By Richard Balmforth
KIEV, Aug 5 (Reuters) - 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.
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.
Government troops have been battling the rebels since April in a war in the Russian-speaking east in which the United Nations High Commissioner for Human Rights says more than 1,100 people including government forces, rebels and civilians have died.
About 730,000 Ukrainians have left the country for Russia this year due to the fighting, the European head of the United Nations agency for refugees said, a far bigger exodus than previously thought.
Ukraine and its Western allies accuse Russia of orchestrating the revolt and arming the rebels - something denied by Moscow. The United States and the European Union have imposed sanctions on Russia.
Fighting has intensified since the shooting down of a Malaysian airliner last month, killing all 298 people on board - an act the West laid at the door of the rebels. Russia and the rebels blame the disaster on Kiev's military offensive.
Ukrainian forces say they have been making steady gains and have virtually encircled the separatists' second-largest stronghold of Luhansk, while rebels have declared a "state of siege" in Donetsk, the largest city they hold.
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.
Kiev's military information center said rebels holding the town of Horlivka, about 100 km (60 miles) from the Russian border, were massing men and equipment to try to break through government lines that threaten to cut their main supply route from the east.
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. (Additional reporting by Pavel Polityuk and Natalia Zinets; Editing by Will Waterman)