Ukrainian troops have pulled back from the strategically and symbolically important airport of Donetsk after days of heavy fighting with pro-Russian separatists.

"The decision was taken to leave the territory of the terminal for new positions," Ukrainian military spokesman Vladislav Seleznyov told Reuters.

The airport, which lies on the northern edge of Donetsk, has been reduced to a wreck by months of clashes and shelling, but held a high symbolic value for Ukrainian forces, as numerous solider died to keep it under government control.

The development came as fighting has flared up over the last few days in the separatist regions of Donetsk and Luhansk, with Kiev accusing Russia of having boosted its military support of the rebels.

Separately, an artillery shell hit a trolleybus stop in Donetsk, killing up to 13 civilians, according to earlier reports.

It was not immediately clear who was responsible for the attack. Ukrainian forces and Russian-backed separatists have been trading blame for a series of similar incidents that killed dozens of civilians.

Last week, 13 civilians were killed as a commuter bus from Donetsk was hit by a shell in the nearby town of Volnovakha.

Ukrainian president Petro Poroshenko claimed the deaths were a direct consequence of Russian heavy artillery fire in support of separatist rebels.

"I have here part of the Volnovakha bus, with the hit of the fragments of the Russian missiles which hit my people. And for me this is a symbol, a symbol of the terroristic attack against my country," he said addressing the World Economic Forum in Davos, Switzerland.

Poroshenko also said some 9,000 Russian troops backed by more than 500 tanks, heavy artillery and personnel carriers are currently inside Ukrainian territory.

"If this is not an aggression, what is an aggression?" he said.

US State Department spokeswoman Jen Psaki also insisted Russian troops have been active inside Ukraine, although she couldn't confirm the figures provided by Poroshenko.

"We've also seen reports that Russia has moved two tactical battalions into Ukraine," said Psaki.

"We can confirm that Russia continues to move tanks, armored vehicles, trucks artillery pieces and other military equipment to deployment sites near the Russia-Ukraine border, which serve as staging points before transporting military equipment to pro-Russia separatists. That is something we're seeing."

Moscow denies any involvement in the conflict, despite evidence suggesting the contrary.