Shelling on Saturday forced international experts to abandon part of the crash scene of the downed Malaysia Airlines jet in east Ukraine as sniffer dogs working elsewhere on the vast site helped uncover the remains of more victims.

Seventy Dutch and Australian police investigators were back for a second day to scour the wreckage strewn over some 20 square kilometres (eight square miles) but members from the probe team hurriedly left a village where some debris was lying after mortar fire nearby.

"We heard at a distance of approximately two kilometres incoming artillery from where we were and that was too close to continue," said Alexander Hug, deputy chief monitor with the Organisation for Security and Cooperation in Europe (OSCE) mission in Ukraine.

"It was close enough to decide to leave. The artillery impact was very loud and the ground was shaking."

Government troops and rebels battling across the war-torn region had pledged to halt fighting around the rebel-held site of the downed Flight MH17 site and Hug said the visit to the village of Petropavlivka had been agreed with both sides.

Closer to the main impact site, Dutch police experts said they managed to recover an unspecified number of remains after deploying two sniffer dogs to help comb through the fields of debris, over two weeks after the plane was shot down killing all 298 people on board.

The perilous security situation highlights the difficulties facing investigators as they try to recover remains and then puzzle together what happened, a grim task that those in charge say could take some three weeks.

The United States accuses insurgents of blowing the airliner out of the sky with a surface-to-air missile likely supplied by Russia, while Moscow and the rebels have pointed the finger at the Ukrainian military.

More than 220 coffins have already been sent back to the Netherlands, which lost 193 citizens in the July 17 crash, but more body fragments remain lying out in the cornfields where the plane came down.

Australian Prime Minister Tony Abbott welcomed the experts gaining access after they were blocked by days of fighting but warned that the search operation would be "a long and slow process".

- 'Humanitarian catastrophe' -

Across the rest of the region the violence that has claimed some 1,150 lives since mid-April raged on.

Ukraine's military said its positions continued to come under heavy fire and that separatists had hit an army drone with a missile similar to the one they say downed MH17.

No new casualties were reported though since an ambush overnight Thursday in a town 25 kilometres (15 miles) from the crash site left 14 people dead, including at least 10 soldiers.

Government forces have made major gains over the past month and say they are getting close to cutting off the main rebel stronghold of Donetsk from the Russian border and a second insurgent bastion of Lugansk.

The fighting has taken a heavy toll on civilians and the mayor in the besieged industrial hub of Lugansk warned that the city was "on the verge of a humanitarian catastrophe" as electricity failed and water and fuel ran low.

- International fallout -

The shooting down of the Malaysia Airlines plane refocused world attention on the conflict and pushed the United States and European Union into imposing the toughest sanctions against Moscow since the Cold War.

In a telephone call with Russian leader Vladimir Putin on Friday, US President Barack Obama expressed his "deep concerns" about Moscow's increased support for the separatist fighters.

Obama said he hoped Putin would heed international pressure to defuse the crisis and help Russia's economy stave off the threat of recession caused by the sanctions.

"But sometimes people don't always act rationally," Obama warned.

The Kremlin said the two leaders had agreed that the standoff was "not in the interest of either country".

Putin -- who views Ukraine's pivot westwards as a fundamental threat to Russia -- lashed out at the latest punitive measures as "counterproductive", damaging bilateral cooperation and international stability.

Some EU diplomats have warned that the sanctions could actually embolden Putin by convincing him he has nothing to lose by going all-in over the Ukraine crisis.

Stoking those fears, the Pentagon and NATO have said Russia was continuing to reinforce its military presence along the border with Ukraine.