WASHINGTON, July 17 -- Evidence from the crash of Malaysian airliner must stay in Ukraine before it is investigated, U.S. President Barack Obama and Ukrainian President Petro Poroshenko said in their telephone conversation Thursday.

"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," said a statement issued by the White House summarizing the talk between the leaders.

A Malaysian flight with 280 passengers and 15 crew members on board crashed Thursday in Ukraine near the Russian border. U.S. officials told media that they concluded the plane was shot down by missile.

Obama assured Poroshenko that U.S. experts will offer all possible assistance immediately, the statement said.

Poroshenko welcomed the assistance of international investigators to ensure a "thorough and transparent" investigation of the crash site, according to the statement.