WASHINGTON (AP) — The Obama administration scrambled Thursday, July 17, to determine what brought down a passenger jet in Ukraine and whether any Americans were killed in an incident that could worsen the already tense conflict near the Ukraine-Russia border.

President Barack Obama made no mention of who might be responsible for the crash of the Malaysia Airlines plane carrying 295 people, but said the incident appeared to be a “terrible tragedy.”

The incident came one day after Obama levied broad economic sanctions on Russia as punishment for its threatening moves in Ukraine. Moscow is widely believed to be supporting pro-Russian separatists fomenting instability near the border, though the Kremlin denies those assertions.

Obama discussed the new sanctions by phone Thursday with Russian President Vladimir Putin. The White House said reports of the downed plane surfaced during that call and Putin mentioned the incident to his American counterpart.

Speaking later during a trip to Delaware, Obama said “the world is watching” the deadly incident.

“It looks like it might be a terrible tragedy,” he said. “Right now we’re working to determine whether there were American citizens on board. That is our first priority.”

Obama said his administration was in close contact with Ukrainian officials and was prepared to assist in determining what caused the crash. Vice President Joe Biden spoke by phone with Ukrainian President Petro Poroshenko, who has called the downing of the plane an act of terrorism.

Uploaded by Chelsea Leonard.