The shock announcement made on Thursday added to an already 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 gravity of the situation facing the country was underscored by allegations from Washington that US has evidence of Russian troops are firing artillery on Ukrainian military positions from Russian soil.

Prime Minister Arseniy Yatsenyuk said he was stepping down over the "dissolution of the parliamentary coalition and the blocking of government initiatives" after several parties walked out on the ruling group.

The collapse of the ruling coalition paves the way for early elections to be called by President Petro Poroshenko within 30 days.

Although a truce has been declared by both rebels and government forces in the immediate vicinity of the vast crash site, heavy shelling was ongoing nearby including around Donetsk, just 60 kilometres (40 miles) from the scene.

Ukraine's army reported four soldiers killed over the last 24 hours in its offensive to retake the eastern industrial heartland from pro-Russian insurgents.

Countries which lost 298 citizens in the disaster are looking to deploy armed police to secure the impact zone, with Dutch Prime Minister Mark Rutte announcing the Netherlands was sending 40 unarmed police to the crash site.

"On the site it is still clear that nothing is happening without the approval of the armed rebels who brought the plane down in the first place," said Australian Prime Minister Tony Abbott, whose country lost 28 citizens in the crash.

"There has still not been anything like a thorough professional search of the area where the plane went down, and there can't be while the site is controlled by armed men with vested interest in the outcome of the investigation."

Abbott has placed 50 Australian officers on stand-by in London.

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