The White House warned on Thursday that evidence from the Malaysian airliner shot down over Ukraine must not be moved from the country until a "thorough and transparent" investigation has taken place.
US President Barack Obama and Ukrainian President Petro Poroshenko agreed in a telephone call on the need to prevent tampering with debris from the Boeing 777 jet that crashed in rebel-held eastern Ukraine after apparently being hit by a surface-to-air missile.
Obama assured Poroshenko that US experts will "offer all possible assistance immediately" to investigate what caused the plane to plunge from the sky with its 298 passengers.
"The presidents emphasized that all evidence from the crash site must remain in place on the territory of Ukraine until international investigators are able to examine all aspects of the tragedy," the White House said.
The statement raised the possibility that US officials are concerned pro-Russian forces could try to tamper with the evidence of the wrecked aircraft to cover up who is to blame.
US officials say they are convinced that the jet, traveling from Amsterdam to Kuala Lumpur, was hit by a surface-to-air missile.
But they have not yet been able to say who is to blame for firing the missile, and from where it was launched.
Obama also spoke by telephone to Malaysian Prime Minister Najib Razak, as his country reeled from the second disaster to strike the national airline within four months.
Malaysia Airlines flight MH270 disappeared on March 8, and is believed to have crashed into a remote part of the Indian Ocean, though no trace of the plane has yet been found.