The airport just outside Donetsk, which is no longer in use, has taken on symbolic value for both separatists and the government. Photo: BBC News

It said the military retained control of parts of the airport, but that six soldiers had died and 16 were wounded.

In Donetsk itself, at least nine people were killed when a civilian bus was hit by shelling.

Fighting between the army and Russian-backed separatists has intensified in and around rebel-held Donetsk.

The deaths come after Ukraine, Russia, France and Germany issued a joint call to end fighting in the east.

They also agreed on a line of demarcation between separatists and government forces from which both sides are meant to withdraw their forces, but so far this has not been carried out.

The airport just outside Donetsk, which is no longer in use, has taken on symbolic value for both separatists and the government.

Ukraine's defence ministry said 20 Ukrainian soldiers had withdrawn from the main terminal because their positions were destroyed and they were under direct shelling.

The loss of the main terminal is a major blow to pro-Kiev forces and will send political shockwaves back to the capital, the BBC's David Stern reports from Kiev.

The defence ministry blamed separatists for the shelling of the bus, saying their own forces were 15km (9 miles) away.

But Russian Foreign Minister Sergei Lavrov said Ukrainian forces were to blame for what he called a "monstrous new crime" and a "coarse provocation".

Some reports, citing local officials, said that as many as 13 people had died.

Last week, 13 people were killed when a bus was struck by artillery fire in Buhas, south-west of Donetsk.

More than 4,800 people have been killed and some 1.2 million displaced since pro-Russian rebels seized parts of Luhansk and Donetsk regions in April.

The rebel movements followed Russia's annexation of Ukraine's Crimea peninsula in March - weeks after Ukraine's pro-Russian President Viktor Yanukovych was ousted.

There have been reports of fighting in recent days near the town of Slovyanoserbsk, north-west of Luhansk - an indication that violence is spreading beyond the Donetsk region.

Ukraine says Russia has more than 9,000 soldiers fighting alongside the rebels, a claim that Moscow denies.

Nato Secretary-General Jens Stoltenberg said on Wednesday there had been a recent build up in the numbers of Russian tanks and other heavy military equipment in eastern Ukraine.

Russian Deputy Prime Minister Arkady Dvorkovich repeated Russian denials of intervention in Ukraine.

"We know that there are some Russians that fight on the side of those regions in Ukraine - some people that are coming on their own and fight," he told the BBC.

"We're trying to make sure that those people [in eastern Ukraine] are protected and to preserve their rights," he said. "We need a peaceful Ukraine."