U.S. President Barack Obama said the downing of a Malaysian jetliner in a Ukrainian region controlled by Russian-backed separatists should be a "wake-up call for Europe and the world" in a crisis that appears to be at a turning point and warned Russia of possible tightening of sanctions.

While stopping short of blaming Russia for Thursday's crash of Malaysia Airlines Flight MH17, in which all 298 people died, Obama accused Moscow of failing to stop the violence that made it possible to shoot down the plane.

The United States has said the jetliner was hit by a surface-to-air missile fired from rebel territory.