Kiev, Reuters—Pro-Russian separatists opened fire on unarmed Ukrainian soldiers on Tuesday as they crossed back into Ukraine from Russia where they had taken shelter from fighting, Kiev defense officials said.

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.

“Today at about 6:00 am [separatist] fighters treacherously fired on a column in which there were 195 unarmed Ukrainian troops returning from the Russian Federation,” Andriy Lysenko, a spokesman for the national defense council, told Interfax news agency. He had no immediate word on casualties.

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.

Ukraine accuses 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 which 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.

Ukrainian fighters carried out air strikes on Monday night, destroying concentrations of rebels and military equipment, Kiev’s military information center said, without giving details or casualty estimates.

As the Ukrainian army recaptures more territory from the separatists, Russia alarmed Kiev on Monday by announcing military exercises near the long joint border.

More than 100 aircraft, including fighter jets and bombers, would take part in the manoeuvres this week in the central and western military districts, the Russian air forces said.

The Ukrainian Foreign Ministry condemned the Russian move on Tuesday and accused Moscow of continuing to arm the rebels.

“We categorically demand from the Russian side to put an end quickly to such criminal actions [and] to pull its troops back from the border with Ukraine,” it said in a statement.

Ukrainian security officials said separatist fighters were launching counter-attacks to break a tightening noose around their positions while Ukrainians troops continued to come under mortar and artillery shelling from inside Russian territory.

Russian military equipment and consignments of arms continued to roll into Ukraine to supply the rebels, the Ukrainians said on Tuesday, though Moscow denied it is providing support for 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 60 miles (100 km) from the Russian border, were massing men and equipment to try to break through government lines which threaten to cut their main supply route from the east.