WASHINGTON, July 18 -- Evidence showed that a Malaysian airliner brought down over Ukraine was shot down by surface-to-air missiles launched from an area that is controlled by separatists in eastern Ukraine, U.S. President Barack Obama said on Friday.

At least one American citizen was among the 298 people killed when the Malaysian Airlines Flight MH17 was brought down over eastern Ukraine on Thursday, Obama said at a press briefing at the White House, adding that "our thoughts and prayers are with his family for this terrible loss."

He urged an immediate cease-fire to allow for a credible investigation into the "outrageous event."

"This was a global tragedy," he said. "An Asian airliner was destroyed in European skies filled with citizens from many countries, so there has to be a credible international investigation into what happened."

The passenger jet, a Boeing 777, was shot down Thursday in Ukraine near the Russian border when it was en route from Amsterdam to Kuala Lumpur. No survivors have been found from the crashed plane. The incident occurred one day after Obama announced broader economic sanctions against Russia for its destabilizing moves in Ukraine.

Obama said the event underscored that it was time for peace and security to be restored in Ukraine.

"Now is, I think, a somber, appropriate time for all of us to step back and take a hard look at what has happened. Violence and conflict inevitably lead to unforeseen consequences," he said.

The president blamed Russia for creating the instability that led to the shootdown of the plane.

"We want Russia to take the path that would result in peace in Ukraine, but so far, at least, Russia has failed to take that path, " he said, adding that the event would be a "wake-up call" for Europe and the world that there are consequences to an escalation of the conflict in eastern Ukraine.

Obama said that personnel from National Transportation Safety Board and the Federal Bureau of Investigation are on their way to Ukraine to assist in the probe.

Also Friday, Pentagon spokesman John Kirby said that U.S. military does not directly know who is responsible for firing the missile or what their intentions are.

He said there is no plan right now for the Defense Department to participate in the investigation.