Pro-Russian separatists in Ukraine have said they will allow safe access to the conflict zone so that international experts can visit the MH17 plane crash site.

A "contact group" of senior representatives from Ukraine, Russia and the Organisation for Security and Co-operation in Europe (OSCE) held a Skype call with the separatists.

In it, the rebels pledged to close down the site, and work with the Ukrainian authorities in the investigation.

Kiev had earlier complained separatists prevented Ukrainian officials from reaching the site where the Malaysia Airlines passenger plane crashed.

In a statement on Friday the OSCE said: "As a matter of priority, they (the separatists) shall close off the site of the catastrophe and allow local authorities to start preparations for the recovery of bodies."

A 30-strong OSCE team is expected to arrive mid-afternoon local time, to "watch, observe and advise" on what equipment and expertise are needed.

Michael Bociurkiw, one of the team members, said: "We’re going to go there, establish the facts, then report on what we’re seeing.

"The situation is in a state of flux most of the time, and there are still a lot of challenges before in-depth observation can happen."

A ceasefire of up to four days is being discussed to facilitate an investigation - a move backed by the US.

Investigators will be looking at various types of evidence, and follow a range of leads, to find out who was to blame.

Both of the the plane's 'black box' flight recorders have reportedly been recovered, with one understood to have been taken to Moscow.

Aviation expert Neil Hansford said: "The black box itself won’t tell us much because the aircraft was operating probably perfectly.

"The interesting thing will be the cockpit voice recorder, which will be the reaction of the cockpit crew when the impact has happened.

"Now, before we all rush in and say ‘absolutely it was a missile’ we need to look at the wreckage to see whether it was a blow out from a bomb on board or a blow in from a missile."

Photographs and videos from the scene, taken just minutes after the crash, will be analysed for clues, as well as the trail left from the missile.

Radar expert Professor David Staples told Sky News: "A missile when it's fired leaves quite a trace behind it.

"It could have been picked up on a military surveillance radar, it could probably be picked up on air traffic radars, and it could also be picked up on military surveillance equipment."

He said it was unlikely that the crew would have known a missile was heading towards the plane.

And social media could also be key.

Sky's Katie Stallard, in Moscow, said Igor Strelkov, the commander of the pro-Russian Donetsk People's Republic, appeared to have boasted about the incident on social media.

In one deleted message recovered by Sky News, he allegedly wrote: "We warned you not to fly over our sky."

Ukraine's security services have also released what it claimed was a recording of an intercepted phone call between two Russian military intelligence officers, discussing the downing of the plane.

Ukraine analyst Andy Hunder, speaking from Kiev, told Sky News: "There have been calls intercepted showing that pro-rebel leaders were boasting to their superiors in Moscow saying: 'We've brought down a plane'."

He said it indicated "Kremlin-backed terrorism".

President Barack Obama has called for a "credible and unimpeded" international inquiry, and Washington has offered help from the FBI and National Transportation Safety Board.