Terming the crash of a Malaysian airliner in eastern Ukraine as a "terrible tragedy", US President Barack Obama said officials were trying to establish if any Americans were on board.
"The world is watching reports of a downed passenger jet near the Russia-Ukraine border, and it looks like it may be a terrible tragedy," Obama said at Wilmington in Delaware, where he delivered remarks on the US infrastructure.
"Right now we're working to determine whether there were American citizens on board. That is our first priority. I've directed my national security team to stay in close contact with the Ukrainian government," he said.
"The United States will offer any assistance we can to help determine what happened and why, and as a country our thoughts and prayers are with all the families of the passengers wherever they call home," Obama said.
Earlier the US President was informed about the plane crash by Russian President Vladimir Putin, a presidential spokesman said.
Earlier he was briefed by his national security team on the accident.
The United States is closely monitoring the situation, State Department Spokesperson Jen Psaki said.
"At this point, we do not have any confirmed information about casualties, the cause or additional details. Our thoughts and prayers go out to those on board, their families and loved ones," she told reporters at her daily news conference.
"Our embassy in Kiev is also in close contact with the Ukrainian authorities on this incident, but at this point those are all the details that we have," Psaki said.