A train carrying the remains of most of the 298 victims of the Malaysia Airlines plane downed over Ukraine left the site on Monday, after the Malaysian prime minister reached a deal with the leader of pro-Russian separatists controlling the area.

The aircraft's black boxes, which could hold information about the crash in rebel-held eastern Ukraine, will be given to the Malaysian authorities, Prime Minister Najib Razak said, indicating he had bypassed Kiev, which has lost control of much of the east.

At the United Nations, the Security Council unanimously adopted a resolution demanding those responsible "be held to account and that all states cooperate fully with efforts to establish accountability".

It also demanded that armed groups allow "safe, secure, full and unrestricted access" to the crash site.

The expected handover of the bodies and the black boxes, and reports by international investigators of improved access to the wreckage of the airliner four days after it was shot down, takes place against calls for broader sanctions against Russia for its support for the rebellion, though Western leaders are struggling to agree on a united response.

The Malaysian leader said he had reached an agreement with the separatists for recovered bodies to be handed over to the authorities in the Netherlands, where the largest number of victims came from.

Dutch Prime Minister Mark Rutte told a news conference that a train carrying about 200 body bags was on its way to rebel-held Donetsk and then to Kharkiv, which is in Ukrainian government hands, from where the bodies would be taken back to the Netherlands to be identified.

The shooting down of the airliner on Thursday sharply deepened the Ukrainian crisis, in which separatist gunmen in the Russian-speaking east have been fighting government forces since pro-Western protesters in Kiev forced out a pro-Moscow president and Russia annexed Crimea in March.

Western governments have threatened Russia with stiffer penalties for its backing of pro-Russian militia who, their evidence suggests, shot the plane down.

But, with Russia challenging them to produce proof, some of those taking a firmer line are saying the acid test will be if the separatists improve access to the site and Russia stops supporting them.

European Union foreign ministers are due to discuss further penalties on Tuesday, but the most they are expected to do is to speed up implementation of sanctions against individuals, and possibly companies, agreed in principle last week before the plane was brought down.

Diplomats say more serious sanctions against whole sectors of the Russian economy will depend largely on the line taken by the Netherlands because of the number of Dutch victims.

Emotions ran high in the Netherlands, where prosecutors opened a war crimes investigation.

"It is clear that Russia must use her influence on the separatists to improve the situation on the ground," Prime Minister Rutte said.

"If in the coming days access to the disaster area remains inadequate, then all political, economic and financial options are on the table against those who are directly or indirectly responsible for that," Rutte said.

U.S. President Barack Obama echoed that approach.

"Now's the time for President Putin and Russia to pivot away from the strategy that they've been taking and get serious about trying to resolve hostilities within Ukraine," he said at the White House.

Putin and Russia have a direct responsibility to compel separatists to cooperate with the investigation, and the burden is now on Moscow to insist that separatists stop tampering with the investigation, he said.

"What are they trying to hide?" Obama said.