The head of the Dutch forensic team in Ukraine says a train carrying the remains of MH17 victims contains significantly fewer bodies than claimed by pro-Russian rebels.

The separatists said there were 282 bodies on the train but only 200 have been found.

"We are sure of having 200 bodies and body parts, that is all that I know," said Jan Tuinder, the head of the Dutch delegation.

Meanwhile, US intelligence officials say evidence suggests MH17 was shot down by "mistake" by ill-trained pro-Russian separatists.

Evidence gathered so far suggests separatists launched the SA-11 surface-to-air missile that blew up the Malaysian airliner, but it remains unclear "who pulled the trigger" and why, said a senior intelligence official who spoke to reporters on condition of anonymity.

"The most plausible explanation ... was that it was a mistake," and that the missile was fired by "an ill-trained crew" using a system that requires some skill and training, the official said.

Dutch team will need to return to crash site

Speaking after the train arrived in the city of Kharkiv, Mr Tuinder said his team would need to go back to the crash site to carry out another search for more bodies.

International monitors say the crash site is still being compromised by separatists near Donetsk, a stronghold of pro-Russian rebels, where fighting with Ukrainian troops flared again on Tuesday.

"There were human remains that had not been picked up," said Michael Bociurkiw, a spokesman for the Organisation for Security and Cooperation in Europe (OSCE) observer mission after visiting the scene, amid reports of the wreckage being rearranged.

"What struck us is that we did not monitor any recovery activity in place," he said, pointing out that OSCE observers saw human remains in at least two areas at the sprawling crash site in rebel-held territory.

Pro-Russian insurgents said Monday they had released the bodies of 282 victims after they were sent by refrigerated train to the government-controlled town of Kharkiv, some 300 kilometres to the northwest.

ABC reporter Phil Williams is in Ukraine and says the discrepancy is "significant".

"I thought [282] was an odd figure when I heard it, because it sounded too high because of the area that had to be searched," he said.

"I saw the method of searching, I thought there is no possible way they can find as many [bodies] as that.

"Now, it appears ... there's still many, many bodies to be recovered."

The train with five refrigerated carriages arrived yesterday and an Interpol forensics team has begun the task of identifying victims.

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Russian president Vladimir Putin has said he would urge the separatists to allow a full investigation.

Western governments have threatened Russia with broader sanctions for what they say is its backing of the militia. However, they are struggling to agree a response, and European Union ministers meeting in Brussels on Tuesday delayed action for a few days.

The Netherlands is leading the investigation, while Malaysia says it will look after the plane's black boxes until a team is set up.

Almost 300 people, predominantly Dutch but also including 37 Australian citizens and permanent residents, were killed when the Malaysia Airlines plane was shot down on Thursday.

Operation Bring Them Home to transfer bodies

Prime Minister Tony Abbott yesterday announced Operation Bring Them Home to secure and identify the bodies of Australian victims.

The Prime Minister's special envoy, Air Chief Marshal Angus Houston, is in Kharkiv and says Australian officials have not yet made it to the crash site.

"I think we are moving pretty quickly. Things appear to have settled down a little bit ... but I would emphasise that there is still extensive fighting in eastern Ukraine and the environment is very fluid," he said.

An RAAF C17 plane carrying the remains of victims so far recovered from the wreckage is expected to depart Kharkiv this evening and travel to the Netherlands.

MH17 intensifies security fears How frequently do airliners fly over conflict zones where anti-aircraft weapons are used? International correspondent Mark Corcoran looks at who makes the decision, and how airlines can prevent missile attacks.

In a statement, the Prime Minister's Office says the plane, as well as a Dutch aircraft, will continue the sombre journey until all remains are transferred.

Governor-General Sir Peter Cosgrove will be present for the arrival of both the Dutch and the Australian aircraft.

"Once the Australian victims of MH17 have been identified, the Government will transport their families to the Netherlands, should they wish, so they can accompany their loved ones home," the statement read.

Operation Bring Them Home involves 100 Australian officials, including consular staff, victim identification and forensic experts and air safety investigators in Ukraine and the Netherlands.

Mr Abbott said when the bodies had been removed, experts would begin the painstaking process of identifying the Australian victims.

"I need to caution that this is necessarily a painstaking and methodical process that will take some weeks," he told reporters in Canberra yesterday.

"As frustrating as this is, we do have to get it right. It would be terrible to compound the families' grief by risking the misidentification of their loved ones."

Warnings identification of victims could take months

Malaysian prime minister Najib Razak spent several days negotiating with rebels to transport the bodies of the MH17 plane victims out of Donetsk.

A small group of Malaysian air crash experts became the first international accident investigators to reach the site on Tuesday, escorted by a convoy of international monitors and heavily armed separatist fighters.

Dutch prime minister Mark Rutte cautioned identification of the bodies could take months.

"As soon as a victim is identified, first and foremost the family will be informed and no-one else. That can take weeks or months," he said.

He confirmed the Netherlands would lead the crash investigation, which normally would have fallen to Kiev, although it does not control the area around the crash site.

ABC/wires