Intense artillery fighting has broken out in the separatist bastion of Donetsk in eastern Ukraine, according to AFP journalists in the area.

Firing close to the town centre began at 2:00am (local time) and could still be heard at the same intensity two hours later.

It is the first fighting on this scale since a barely-observed ceasefire was signed in September.

The shelling came after the Organisation for Security and Cooperation In Europe (OSCE) said it was "very concerned" after its monitors witnessed tanks and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.

In a statement on Saturday, the OSCE said it had observed convoys of heavy weapons and tanks in the area of Donetsk and nearby Makiivka.

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.

Moscow denies being involved in the fighting, although it openly gives the pro-Russia separatists political and humanitarian backing.

"More than 40 trucks and tankers" were seen driving on a highway on the eastern outskirts of Makiivka, the OSCE representatives said.

"Of these, 19 were large trucks - Kamaz type, covered, and without markings or number plates - each towing a 122mm howitzer and containing personnel in dark green uniforms without insignia. Fifteen were Kraz troop carriers," the report said.

There were also six fuel tankers and an armoured personnel carrier.

Separately, the OSCE monitors said they had seen "a convoy of nine tanks moving west, also unmarked," just south-west of Donetsk.

The OSCE said all the forces were on territory controlled by the self-declared Donetsk People's Republic, which is fighting Ukrainian troops in a war that has already killed at least 4,000 people, according to UN figures.

The Swiss foreign minister and OSCE chairperson-in-office, Didier Burkhalter, said he was "very concerned about a resurgence of violence in the eastern regions of Ukraine".

"He urged all sides to act responsibly and to do all in their power to further consolidate the ceasefire," the OSCE said.

Despite a ceasefire signed in September, there is daily shelling across the separatist zone and a steady loss of life among soldiers and civilians.

Meanwhile, the former Soviet leader Mikhail Gorbachev has warned tensions over the Ukraine crisis were threatening to push the world into a new Cold War.

Speaking at an event to mark 25 years after the fall of the Berlin Wall, Mr Gorbachev urged Europe and Russia to settle their differences.

AFP