A senior pro-Russian rebel commander in eastern Ukraine has suggested that many of the victims of the MH17 plane crash may have died days before the plane took off.

Igor Girkin said, on the pro-rebel website Russkaya Vesna, that he was told by people at the crash site that a "significant number" of the bodies were not fresh.

The alleged former Russian military intelligence agent added that he was told the bodies of the victims were drained of blood and reeked of decomposition.

Girkin, who is also known as Strelkov, said he could not confirm the bizarre claim but added: "Ukrainian authorities are capable of any baseness." He also said that a large amount of blood serum and medications were found at the site of the wreckage.

The Malaysian Airlines MH17 jet was 'shot down' over a rebel-held part of eastern Ukraine on Thursday while en route from Amsterdam to Kuala Lumpur. All 298 people on board died, including 80 children.

Pro-Russian rebels have been accused of looting valuables belonging to victims of the crash and attempting to destroy any criminal evidence linking the crash to the separatists.

International observers said pro-Russian rebels have also hampered their access to the wreckage of the plane.

Michael Bociurkiw, a spokesman for the European security watchdog OSCE, said his team's access to the crash site had been limited despite assurances from the regional rebel commander that they would be allowed into the area.

"A visibly intoxicated armed guard fired his rifle in the air when one of the observers walked out of the prescribed area," Bociurkiw told journalists.

The 25 observers withdrew after just over an hour when they were unable to set up an access corridor for specialist teams to investigate the crash.

The pro-Russian self-proclaimed Donetsk People's Republic has denied reports that an agreement was reached between Kiev and pro-Russian rebels to set up a 20km (20-mile) security zone around the wreckage.

Australian Prime Minister Tony Abbott said it is vital for investigators to have full access to the site as it is likely that there are attempts to "sanitise" the crash scene.

"The site needs to be secure," he said. "The investigation needs to take place and frankly anyone who tries to obstruct this is no friend of justice and is no friend of peace."