Intense artillery fire has broken out in the pro-Russia separatist bastion of Donetsk in eastern Ukraine, the worst since a barely-observed ceasefire was signed in September.

Shelling by heavy artillery continued throughout the night and into the early hours of Sunday, and then picked up again later in the morning.

The bombardment appeared to come from areas held by the separatists as well as from positions controlled by the government forces, and could be heard in the centre of the city, which had a pre-conflict population of more than 1 million.

The truce, agreed on September 5, has looked particularly fragile over the past week, with each side accusing the other of violations after separatist elections on November 2 that were condemned as illegitimate by the West.

Residents said there appeared to be fighting near Donetsk airport, around which battles have repeatedly violated the ceasefire in a conflict in which around 4,000 people have been killed since mid-April.

There were no immediate reports of new casualties. Donetsk's City Council said in a statement on its website that the shelling had damaged residential buildings and that firefighters were at the scene of one of the damaged buildings.

The Organisation for Security and Cooperation in Europe (OSCE) voiced concern on Saturday after its monitors witnessed columns of tanks, howitizers and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.

The OSCE report came a day after Ukraine's military said it had spotted a large column of tanks and other heavy weapons entering the country from Russia across a section of border that has fallen under control of rebel fighters.

Russia has denied it was backing the rebels in the east.