Gunmen have prevented international investigators from securing an access corridor to the wreckage of a Malaysia Airlines plane that was shot down in eastern Ukraine.

Twenty-eight Australians died on board flight MH17 which was shot down by a ground-to-air missile, killing all 298 passengers and crew.

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

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.

"They did not have the freedom of movement that they need to do their job," OSCE chairman Thomas Greminger told Reuters by telephone.

"The crash site is not sealed off. In the current circumstances, they were not able to help securing this corridor that would allow access for those that would want to investigate."

He also told a press conference that full access was blocked by "local, illegally-armed groups present on the ground".

An OSCE spokeswoman said shots were fired during the visit, but said OSCE monitors were not shot at and no-one was hurt.

Michael Bociurkiw, who was part of the OSCE's special monitoring mission in Ukraine, said the separatists were aggressive, unprofessional and seemingly affected by drugs and alcohol.

He said while his team was unable to complete its work, the visit was not a complete waste.

"We were able, for example, to establish that there is very little security around the perimeter of the crash site which could extend for up to six kilometres," he said.

"It appears to us the bodies weren't being moved, they weren't being tampered with.

"There's no apparent tampering of the debris - you have some fairly large pieces in the field. It's astonishing to go there. [There is] no recovery going on, with these bodies starting to decompose."

Mr Greminger, who is Switzerland's ambassador to the European rights and security watchdog, said the team was now returning to Donetsk.

He said the team would try again on Saturday (local time).

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

The Ukrainian government says it has evidence three people were operating the missile launcher that brought down the plane.

According to the Ukrainian government, one of those arrested has documents suggesting he had expertise in missile technology.

Russian president Vladimir Putin has vehemently denied Russian involvement, accusing the Ukrainians of fostering violence in the area.

He says he will co-operate fully with any inquiry.

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

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

Questions over 'national competence' and sovereignty

A pro-Russian separatist holds a stuffed toy found at the crash site ( Reuters: Maxim Zmeyev )

In principle, all sides support a call, backed by Russia and other world powers in the UN Security Council, for an impartial international investigation.

But even agreeing the Kiev government has jurisdiction in a region where separatists have declared their own republic poses difficulties.

"It is a national competence, and that is part of the problem in this case," said Roland Bless, spokesman for the Swiss chair of Europe's OSCE security body, referring to rebel claims to sovereignty in the eastern region of Donetsk.

In a typical crash inquiry, it is up to Ukraine, on whose territory the plane came down, to secure the area and recover the flight data and cockpit voice recordings and liaise with the manufacturer to ensure their contents are downloaded correctly.

Meanwhile, Ukrainian president Petro Poroshenko has spoken with other world leaders about the running of an international investigation, which could allay some of the rivalries.

"It's very important that unbiased international experts will be the first persons who get access to the black boxes," said Ukraine's UN ambassador in Geneva, Yurii Klymenko.

"The issue is who will ... open the boxes? We would like to have the true information, not the fake [information]."

London lawyer Jim Morris from Irwin Mitchell, a former British military pilot, said if another body obtains the recorders it must hand them over, otherwise it will run the risk of corrupting the data or being suspected of trying to destroy it.

After a Soviet fighter shot down Korean Air Lines flight KAL 007 in 1983 when it strayed into Soviet air space, Moscow found the black box - but only handed it to international investigators after the end of the Cold War nearly a decade later.

The United Nations air safety arm, the International Civil Aviation Organisation, said it has made an offer to Ukraine to put together a team of international investigators.

Black box may contain little information about missile

Aviation attorney Brian Alexander, from New York law firm Kreindler and Kreindler, said that despite the "dicey" situation in the area, "with the international pressure, the community should be able to get a team to the site and allow an investigation to proceed as normally".

"But trying to figure out who provided the training, the weapons, that's a much thornier issue," he said, adding that investigators would be looking for debris not part of the plane.

However, the flight data may offer little information on what caused the Boeing 777 to come down.

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

An explosion by a heavy missile that blew the aircraft apart could show only as a sudden, catastrophic collapse of all the onboard systems.

If, as Ukraine alleges, MH17 was hit by an SA-11 Buk missile, there is a good chance the pilots did not see it coming, leaving little or no informative trace on the cockpit voice recording.

The wreckage might show traces of explosives that could indicate the blast of a warhead.

Somewhere in the debris strewn for miles across the steppe might be remnants of a missile.

But finding them will be hard and proving their provenance after the confusion and mutual accusations of the first day will be tough.

A former British airman said: "Any recovered missile fragments could be analysed. But unless there is a stark difference in the exact type of arms both sides hold, differentiating is not easy."

In fact, all sides use similar, former Soviet hardware.

Washington security expert Anthony Cordesman, from the Center for Strategic and International Studies, wrote in a report that while the use of an SA-11 or SA-17 radar-guided missile seemed likely, determining whether it was fired by pro-Russian rebels or by Ukrainian or Russian armed forces would be problematic.

Mr Cordesman suggested that any of these three parties might have opened fire in an area of high tension, believing their target to be hostile rather than a civilian aircraft.

Noting in 1988 that the US navy shot down an Iranian airliner that Washington said was mistaken for a warplane, he said: "Human error does happen, particularly when both sides may be on the edge of overreacting and have virtually no real operational experience."

"The fact this is a horrible human tragedy should not lead to rushed judgment as to motive, guilt, or intent."

A map shows the path of Malaysia Airlines flight MH17, which was shot down over Ukraine on July 17, 2014, killing 298 people. ( ABC News )

ABC/Reuters