The Ukrainian government says it has evidence three people were operating the missile launcher that brought down Malaysia Airlines flight MH17 over rebel-held territory near the Russian border.

Twenty-eight Australians died on board the passenger jet which was shot down by a ground-to-air missile over eastern Ukraine, killing all 298 passengers and crew.

Authorities in Kiev say two men have been captured near the border on the Ukrainian side, one of whom reportedly had paperwork indicating he was a missile specialist.

Pro-Russian separatists are being blamed for the attack, which left bodies and smouldering wreckage scattered across fields.

But Russian president Vladimir Putin has denied the accusations, saying Ukrainian forces are responsible because they continue to fight the rebels instead of negotiating.

Mr Putin called for a ceasefire in Ukraine's east to allow for negotiations.

"It is with great concern and sadness that we are watching what is happening in eastern Ukraine. It's awful, it's a tragedy," Mr Putin said at a meeting with Russian Orthodox Church leaders.

"All sides in the conflict must swiftly halt fighting and begin peace negotiations."

Ukrainian prime minister Arsney Yatsenyuk has called on the international community to support moves to catch who he called "those bastards" responsible for the attack.

He wants the tragedy treated as a war crime in The Hague.

Many of the bodies remain where they fell, and there are unconfirmed reports of some looting, as fighting continues unabated in eastern Ukraine.

International investigators have been unable to secure an access corridor to the wreckage of a Malaysia Airlines plane that was shot down in eastern Ukraine.

A team of 17 investigators from the Organisation for Security and Cooperation in Europe were allowed to examine the wreckage for about 75 minutes, before being forced out by gunmen.

It is understood officials are negotiating with rebels to secure a safe passage to the Ukraine crash site, with Ukrainian emergency services reportedly finding two black boxes at the crash site.

Interfax-Ukraine quoted an adviser to the governor of eastern Ukraine's Donetsk region as saying the black boxes had been located.

"Two black boxes were found by our emergency services. I have no information on where these boxes are at the moment," Kostyantyn Batovsky said.

However, earlier reports suggested pro-Russian separatists were in possession of the black boxes.

Obama calls for investigation, immediate ceasefire

US president Barack Obama said a surface-to-air missile fired from territory controlled by Russian separatists in Ukraine brought down the jet, adding that it was too early to determine whether the airliner was deliberately targeted or mistakenly shot down.

Mr Obama called for an immediate ceasefire and a "credible, international investigation" into the incident.

Nationalities of MH17 passengers Netherlands: 189*

Malaysia: 44 (including 15 crew)

Australia: 28**

Indonesia: 12 (including 1 infant)

UK: 9

Germany: 4

Belgium: 4

Philippines: 3

Canada: 1

New Zealand: 1

Unverified: 3 *N.B. One victim was a dual US citizen **N.B. DFAT says 28 Australians were on board; Malaysia Airlines says 27 Australians were on board Source: Malaysia Airlines

"I think it's too early for us to be able to guess what the intentions of those who might have launched this surface-to-air missile might have had," he said.

"What we have confidence in saying right now is that a surface-to-air missile was fired, and that's what brought the jet down."

US representative to the UN, Samantha Power, told an emergency session of the Security Council the Malaysia Airlines jet was likely shot down by a SA-11 Buk missile.

"Because of the technical complexity of the SA-11, it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel," she said.

"Thus we cannot rule out technical assistance from Russian personnel in operating the systems.

"If indeed Russian-backed separatists were behind this attack on a civilian airliner, they and their backers would have good reason to cover up evidence of their crime.

"Thus, it is extremely important that an investigation be commenced immediately."

Australia's deputy representative to the UN, Philippa King, said those responsible for the attack must be brought to justice.

"There must be immediate preservation of the crash site and unfettered access for independent investigators," she told the Security Council.

"Safe access must also be provided to allow for the recovery of the bodies of those who have lost their lives in this tragedy."

Abbott demands full inquiry and Russian cooperation

Earlier, Prime Minister Tony Abbott said he was not accepting Russian explanations for the downed plane and demanded Moscow allow a full inquiry into the tragedy.

Mr Abbott says it appears Russian-backed rebels shot the plane down.

"These were innocent people going about their lives and they have been wantonly killed by Russian-backed rebels, quite possibly, maybe probably using Russian-supplied equipment," Mr Abbott said.

"It really is a test for Russia ... how transparent and fair dinkum is it going to be," Mr Abbott said.

"There can be no excuses, no buck-passing, no blame-shifting."