Outraged world leaders heaped pressure on Russia Sunday to press Moscow-backed rebels in eastern Ukraine to allow investigators proper access to the crash site of the Malaysian MH17 jet.

Rescue efforts were in disarray as armed gunmen in fatigues and balaclavas refused to allow international monitors full access and the grisly remains of some of the 298 passengers killed have yet to be removed.

Ukraine has warned that the rebels, who Kiev and the US have accused of blowing the plane out of the sky with a missile, were "hours away" from removing key evidence across the Russian border.

 As fears grew that evidence was being tampered with world leaders voiced their frustration with Moscow, pushing East-West ties to crisis point after months of discord over the Kremlin's interference in ex-Soviet Ukraine.

US Secretary of State John Kerry told Russian counterpart Sergei Lavrov that Washington was "deeply concerned" investigators were denied "proper access" to the crash site for a second consecutive day.

His spokeswoman Jen Psaki later said the "unacceptable" insecurity at the crash site was an "affront to all those who lost loved ones and to the dignity the victims deserve".

"The United States is also very concerned about reports that the remains of some victims and debris from the site are being tampered with or inappropriately removed from the site," the State Department said.

Kerry was backed by leaders from Britain, the Netherlands, Malaysia, Australia and France -- as well as Ukraine -- in calling on Russian President Vladimir Putin to intervene in getting an international probe under way. (AFP)