By Natalia Zinets and Richard Balmforth

KIEV, Aug 4 (Reuters) - Ukraine said on Monday it was in talks with Moscow over the return of 311 Ukrainian soldiers and border guards who had been forced by fighting with separatists to cross into Russia, but Russian border authorities said the troops were seeking asylum.

Both sides seemed set to use the fate of the troops to score propaganda points as Ukrainian government forces extended steady gains it has made against the pro-Russian separatists since a Malaysian airliner was downed over a rebel-held area on July 17.

Ukrainian defense spokesman Andriy Lysenko said a group of soldiers and border guards, who had been caught between the Russian border to the east and rebel positions in the west, had crossed into Russia in the early hours of Monday.

He put their number at 311, telling a news briefing they had retreated into Russia for safety reasons after helping their comrades break through rebel lines.

Kiev, he said, was now negotiating with Russian authorities for their return.

In Moscow, authorities acknowledged that Ukrainian troops had crossed into Russian territory - though they put the number at 438 - and Russian border guards said they had crossed during the night seeking asylum.

"They were tired of the war and wanted no further part in it," Vasily Malayev, spokesman for the border guards in the Rostov region of Russia, told Reuters by telephone. However, he added that 180 would be returned to Ukraine later on Monday.

Russian Foreign Minister Sergei Lavrov, speaking to Itar-Tass and Rossiya24 TV, said Russia would facilitate the return of the Ukrainian soldiers but suggested they would be vulnerable to prosecution for desertion once they returned home.

"I expect Ukrainian authorities to understand that it is absolutely unacceptable, when Ukrainians ... are forced to fight with their own people, to treat those who refuse to do so as traitors to the motherland," Lavrov said.

RAILWAY HUB

Ukrainian government forces recaptured the important railway hub of Yasynuvata on Sunday from the rebels, Lysenko said, adding that five soldiers had been killed and 15 others wounded in fighting in the 24 hours up to Monday.

However, the rebels, who seized Yasynuvata - which lies just north of the city of Donetsk - in May, denied that government forces had retaken the railway hub.

Government troops, who have been battling the rebels since April, have now all but encircled the separatists' second-largest stronghold of Luhansk and rebels have declared a "state of siege" in Donetsk, the largest city they hold.

Luhansk has been left without electricity or running water and the mobile network is also down, local officials said.

At the crash site of the Malaysian Boeing, international experts resumed their recovery and investigative work on Monday.

"After a delay due to fighting, the team of Australians, Dutch and Malaysian experts began work," a statement by the Dutch mission said.

A large group of international experts began work there on Friday after a lull in fighting in the area allowed them finally, after lengthy delays, to reach the site. The victims included 196 Dutch, 27 Australians and 43 Malaysians. (Additional reporting by Alexei Anischuk in Moscow and Anthony Deutsch in Amsterdam, Writing by Richard Balmforth and Gabriela Baczynska; Editing by Gareth Jones)