WASHINGTON, July 17 -- The United State on Thursday called for an immediate ceasefire in Ukraine to facilitate the probe of a doomed Malaysian airliner.

"We urge all concerned -- Russia, the pro-Russian separatists, and Ukraine -- to support an immediate ceasefire in order to ensure safe and unfettered access to the crash site for international investigators and in order to facilitate the recovery of remains," the White House said.

A Malaysian flight with 283 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 missiles.

It is vital that "no evidence be tampered with in any way and that all potential evidence and remains at the crash site are undisturbed," White House spokesman Josh Earnest said in a statement.

He called for "a full, credible, and unimpeded international investigation as quickly as possible," highlighting the role of international organizations such as the United Nations and the Organization for Security and Cooperation in Europe (OSCE) in Ukraine.

The White House spokesman said the United States is "shocked" by the downing of Malaysian Airlines Flight 17, and offers its deep condolences to all those who lost loved ones on board.

He said the United States continues to seek information to determine whether there were any American citizens on board.

Earnest added that the United States remains prepared to contribute immediate assistance to any international investigation, including through resources provided by the National Transportation Safety Board and the Federal Bureau of Investigation.