With evidence mounting that a surface-to-air missile brought down flight MH17 in Ukraine, experts warned Tuesday that proof remains a long way off and only satellite images can identify who pushed the button.

Recently published photographs show a piece of fuselage from the Malaysian Airlines plane peppered with "a fairly dense but also widespread shrapnel pattern" typical for the blast from an SA-11 surface-to-air missile, said defence analyst Justin Bronk.

Evidence already points to an SA-11 surface-to-air missile having shot down the Boeing 777 over rebel-held Ukrainian territory.

"The shrapnel damage on the airframe parts that's been seen so far is consistent with what you would expect to see from an SA-11 warhead exploding in close proximity," said Bronk, analyst in Military Science at the Royal United Services Institute in London.

"But to get a conclusive answer you would have to take the aircraft away and completely reconstruct it as best as you could," he told AFP, as happened with the wreckage of Pan-Am flight 103 after it was blown up over Scotland in 1988.

The Malaysian plane came down in a war zone on Thursday, killing all 298 people on board, 193 of them Dutch.

While forensics experts tasked with identifying the bodies have arrived at the scene, those probing the cause of the crash are still in Kiev "for security reasons," the Dutch Safety Board leading the probe said.

Rebels, civilians and journalists have been walking all over the crash scene, handling objects, while observers from the Organisation for Security and Cooperation in Europe have said some pieces of wreckage have been sawn in half.

Separatists on Tuesday promised to give investigators free access to the site, and a ceasefire has been declared in the immediate area.

- Huge crash site -

Adding to the mammoth task, crash debris is spread over an area of around 12 square miles (35 square kilometres) after the plane disintegrated at an altitude of 33,000 feet (10,000 metres).

The Dutch Safety Board refused to say if or where the plane fuselage might be reconstructed.

"People tend to reduce air accident investigations to the black boxes. But while they're essential, they are not enough," said former test pilot and air crash investigator Robert Galan.

"For instance, in the case of a missile, examining the wreckage allows you to determine whether the impact came from in front or behind the plane," he told AFP.

But, he warned, it is essential for investigators to gain swift access to the site.

"If people have been walking over essential evidence for three days, you will never find it," Galan said.

- Traces of explosives -

Investigators will also be looking for pieces of shrapnel or traces of explosives from the missile, which likely exploded a few metres away from the plane to maximise damage.

"However, it is worth remembering, that this crash site does sit in an active war zone. So, there's potential for contamination by explosive residue from other munition sources in the area," said Bronk.

Pro-Russian separatists have been accused of firing a Russian-supplied missile, while Moscow says Kiev shot down the Boeing 777.

Alleged conversations between rebels discussing the shooting down, intercepted by intelligence agencies, as well as satellite images around the time of the crash will be key to identifying who ordered the attack and who pushed the button.

But, said Bronk, "the Russian and the separatist side will always just claim that it is a fake."

"The really conclusive thing will be if eventually satellite imagery is released, showing exactly where the missile launcher was and fired. And also, crucially, where it went after that."

Washington, one of the first to suggest a surface-to-air missile was used, has said it has satellite imagery of the missile launch.

Ben Moores, aeronautical expert at IHS Jane's, said the sound recordings on the black boxes, now in the hands of Malaysian authorities, could help prove what kind of missile was used.

"An air-to-air missile makes a small blast, while a ground-to-air missile makes a much bigger one," he said.