Ukraine's separatist bastion of Donetsk came under intense artillery fire early Sunday, in a level of combat not seen in the region since a barely-observed ceasefire was signed in September.

Firing close to the centre of Donetsk began to build up at 2:00 am (2300 GMT Saturday), and could still be heard at the same intensity two hours later, AFP journalists in the area said.

However it was not immediately possible to evaluate the consequences of the artillery bombardment due to an overnight curfew imposed by the rebel authorities in Donetsk and nearby Makiivka.

The Organisation for Security and Cooperation in Europe (OSCE) voiced concern Saturday after its monitors witnessed unmarked columns of tanks 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 denies being involved in the fighting in the east.

However, it openly gives the rebels political and humanitarian backing and it is not clear how the insurgents could themselves have access to so much sophisticated and well maintained weaponry.

Last March, Russian soldiers without identification markings took over the southern Ukrainian region of Crimea, and Moscow annexed the region shortly after.

The OSCE reports from the east came as fears mounted of a total breakdown in the ceasefire signed in September in a bid to end the war, which has killed some 4,000 people, according to UN figures.

Ukraine's military said there was bloody fighting on Saturday around Donetsk international airport, where government forces are defending a pocket of territory near the biggest rebel-held city.

Ukraine on Saturday reported eight of its soldiers killed within 24 hours. Seventeen other soldiers were wounded in shelling of government positions around the conflict zone, according to updates from the military.

Several military columns have been seen by foreign journalists in the east in recent weeks, and Ukrainian officials regularly accuse Russia of covertly deploying troops.

However, these reports are often dismissed by Moscow and by the separatist leaders as inaccurate or invented, while the OSCE's statement was important because it is likely to carry more weight.

"More than 40 trucks and tankers" were seen driving on a highway on the eastern outskirts of Makiivka, the OSCE representatives, who are in Ukraine to monitor the two-month ceasefire, 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.

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 these forces were on territory controlled by the separatists' self-declared Donetsk People's Republic.

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", and urged all sides to act responsibly.

 Meanwhile, the Dutch foreign minister said Saturday that the remains of the last nine victims of flight MH17 may never be recovered from the Ukrainian battlefield where their plane was shot down four months ago.

Foreign Minister Bert Koenders made the grim assessment in the city of Kharkiv, where he attended a memorial service for five more sets of human remains collected from the site of the disaster and flown to the Netherlands.

"We cannot say at this moment in any certain way... at what moment, and even if, we can recover the last nine" victims, he said of the air crash that killed all 298 on board, including 193 Dutch.

The shooting down of the Malaysia Airlines Boeing 777 on July 17 was one of the worst tragedies of a war. So far, the remains of 289 of those victims have been identified.

Ukraine and the West blame Russian-backed separatist fighters using surface-to-air missiles for the catastrophe, while Moscow has pointed the finger at Kiev's forces, in an incident that galvanised international shock over the chaos in a country bordering the European Union.

The conflict has sent relations between Western backers of Ukraine and Russia to their lowest level in decades.

"The world is on the brink of a new Cold War. Some are even saying that it has already begun," the last Soviet leader, Mikhail Gorbachev, said at an event marking the 25th anniversary of the fall of the Berlin Wall.

Russia's economy is suffering from European Union and US sanctions imposed in response to Moscow's support for the separatists. With Russia welcoming last week's rebel elections, which were billed as boosting the separatists' claim to independence, the sanctions look set to remain in place -- and possibly be reinforced.

"We see no reason to lift any sanctions," Koenders said in Kiev, calling the rebel elections "illegal".