KIEV — Ukrainian government forces recaptured an important railway hub from pro-Russian rebels in the east of the country, with five soldiers killed and 15 others wounded over the last 24 hours, a security official in Kiev said on Monday.

A rebels' Facebook page dismissed government forces taking Yasynuvata as a lie.

Kiev has intensified its campaign against the Moscow-backed separatists and made steady gains since a Malaysian airliner was downed over rebel-held territory on July 17.

Government troops have now all but encircled the rebels' 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 on Monday.

"Units taking part in the anti-terrorist operation yesterday took the town of Yasynuvata, which is an important hub of the region's railway system," Andriy Lysenko, a spokesman for Kiev's military operation in the east, told a media briefing.

Separatists seized Yasynuvata — just north of Donetsk nearby a main road leading to Luhansk — in May.

Lysenko also said a group of Ukrainian soldiers and border guards, who have been stuck between the Russian border to the east and pro-Russian rebel position in the west for more than three weeks, crossed into Russia in the early hours of Monday.

"Today at 4:30 a planned operation took place to unblock a Ukrainian military unit nearby Chervonopartyzansk. A large part (of the unit) successfully ... reached a safe place," he said.

"Part of the soldiers (group) went into the Russian territory," he said, noting that Kiev was now trying to negotiate their return. He refused to say how many servicemen had crossed the border but the wife of one of them, speaking on Ukraine's 112 TV channel, put the number at about 200.

On the Malaysian Boeing crash site, international experts started 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 has worked on the site since Friday after fighting in the area had stalled efforts to reach the site for several days before that. The victims included 196 Dutch, 27 Australians and 43 Malaysians.