KHARKIV, Ukraine (AFP) - The remains of the last nine victims of flight MH17 may never be recovered from the Ukrainian battlefield where their plane was downed four months ago, the Dutch foreign minister said Saturday, as fighting rumbled on in the east of the country.

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 downing of the Malaysia Airlines Boeing 777 on July 17 was one of the worst tragedies of a war in which an estimated 4,000 people have died.

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.

Ukraine reported more bloody fighting overnight, with six soldiers killed in the last 24 hours, as Moscow denied claims it had sent tanks across the border.

Ukraine's military said one of those killed was a paratrooper shot by a sniper in Donetsk international airport, where government forces are defending a pocket of territory near the biggest rebel-held city. Fifteen other soldiers were wounded in shelling of government positions around the conflict zone, a statement said.

The fighting rumbled on in the industrial east despite a two-month-old ceasefire deal that has halted significant offensives, but failed to stop shelling at strategic flashpoints.

Donetsk's city hall said two civilians were wounded by shrapnel as the north of the city was rocked by explosions overnight.

A brief morning calm ended with the sound of frequent artillery explosions near the airport, an AFP journalist said.

Two tanks and two armoured fighting vehicles could be seen on the outskirts of the city, while rebels were digging trenches.

"The shooting is getting closer and closer, with heavier weapons. Yesterday I almost jumped out of my bed the bombardments felt so close," said Lyudmila, deputy director at a local school.

Russia, which annexed Ukraine's Crimea region in March and lends close political and humanitarian support to the separatist areas in the east, denied the latest Ukrainian allegation that it was dispatching regular troops to join the fighting.

Ukraine's military made headlines around the world on Friday with the claim that columns of hardware, including 32 tanks, had poured across the border which is under the control of Russia and the pro-Russian rebels.

However, Foreign Minister Sergei Lavrov laughed off the allegation after US State Department spokeswoman Jen Psaki said she had no "independent confirmation" of the report.

"If Psaki doesn't have it, I don't," Lavrov told journalists with a chuckle in Beijing, where he met his US counterpart John Kerry ahead of an Apec summit.

Psaki said that heavy Russian weaponry had been seen on Thursday at a rail yard about 25km inside its border. And journalists have on numerous occasions met Russian soldiers in Ukraine or seen long columns of heavy artillery and troop transports on roads.

However, both the Ukrainian and separatist spokesmen have frequently issued dramatic claims about the war without providing evidence.

The conflict has sent relations between Western backers of Ukraine and Russia to their lowest level since the Cold War.

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, new sanctions could be coming.

A flurry of diplomatic activity is approaching, with the Apec summit in China and a Group of 20 summit in Australia next week, where President Vladimir Putin will have the chance to put his case before world leaders.

Speaking in Beijing, Lavrov appeared to soften Russia's position, saying that US involvement in attempts to resolve the crisis would be a "step in the right direction".

"Our positions on what is happening in Ukraine do not correspond with the United States, but if Washington is interested in contributing to the reconciliation of the situation and creating dialogue between Kiev and the rebel leadership... I think that would be a step in the right direction," the Russian foreign minister said in comments shown on Russian state television.

The United States should discourage "hot heads" in the Ukrainian government from resuming a full-scale conflict with the pro-Russian separatists, Lavrov added.

But in comments marking the 25th anniversary of the fall of the Berlin Wall, US President Barack Obama said, "As Russia's actions against Ukraine remind us, we have more work to do to fully realise our shared vision of a Europe that is whole, free and at peace."