Two military officers look at a piece of debris from the crashed flight MH17 on Friday in Donetsk, Ukraine. Photo: Cui Meng/GT

Ukraine sought Friday to avoid a political crisis after the shock resignation of its prime minister, as fighting between the army and rebels close to the Malaysian airliner crash site claimed over a dozen more lives.

President Petro Poroshenko called on parliament to heed "cold reason" and pass a vote of confidence in the government, a day after Prime Minister Arseniy Yatsenyuk walked out in fury over the collapse of his ruling coalition.

Yatsenyuk's resignation piles on more woes for a country already struggling to cope with a chaotic situation in the rebel-controlled east, where international experts are carrying out a complex investigation into last week's downing of Malaysia Airlines flight MH17 that left 298 dead.

The grave challenges facing the country, where the UN said 230,000 people have fled the fighting, go beyond its borders, as Washington accused Russian troops of firing artillery across the border on Ukrainian forces.

The US has already accused Moscow of supplying the missile system which it believes was used by pro-Russian separatists in east Ukraine to shoot down MH17. It said late Thursday it had evidence that Russia was planning to "deliver heavier and more powerful multiple rocket launchers" to the rebels.

Both Moscow and the rebels deny having anything to do with the shooting down of the passenger airliner and have both promised to cooperate with an international probe into the disaster.

A truce has been declared in the vicinity of the vast crash site in rebel-held Grabove, where experts say some remains of the victims still lay decomposing under the sweltering summer heat more than a week after the tragedy.

Arriving in seven vehicles protected by security personnel and local police, investigators from the Netherlands and Australia Friday searched the crash site for hours.

According to the Organization for Security and Co-operation in Europe special monitoring mission to Ukraine spokesperson Michael Bociurkiw, investigators have found the largest piece of debris so far, a chunk of the aircraft that might belong to the economy cabin.

Human remains were also discovered, Bociurkiw said.

Dutch authorities have said they are only sure that about 200 of the bodies have been recovered from the scene, as two more planes carrying 74 more coffins left Ukraine for the Netherlands.

The Ukrainian government's offensive to regain control of Ukraine's eastern industrial heartland was given a boost Friday when its forces took the strategically-important city of Lysychansk.

A Russian security official said up to 40 mortar shells fired by Ukrainian forces fell on the Russian province of Rostov near the border with eastern Ukraine. Russia said the shells targeted at Russian law enforcement officers.

At the same time, Ukrainian government reported losing 13 soldiers in the past 24 hours, while local authorities in the region of rebel strongholds of Donetsk and Lugansk said 16 people have been killed.

The bloody insurgency has forced 230,000 people to flee their homes, the UN said, including 130,000 who have sought refuge in Russia.

Also on Friday, EU ambassadors reached a preliminary agreement to push ahead with hard-hitting economic sanctions against Russia over the Ukraine crisis but details remained to be worked out, diplomats said.

Key measures suggested by the Commission included closing EU capital markets to state-owned Russian banks, an embargo on arms sales to Moscow and restrictions on the supply of energy and dual-use technologies.

They would not affect current supplies of oil, gas and other commodities from Russia, diplomats said.

Agencies contributed to this story