WASHINGTON — President Obama said Friday that the United States believed that a Malaysian jetliner was shot down by surface-to-air missiles from an area inside Ukraine that is controlled by Russian-backed separatists.

In remarks at the White House, Mr. Obama said the nearly 300 lives taken in the plane crash were an “outrage of unspeakable proportions” and a “global tragedy.” He vowed to investigate exactly what happened to end the lives of “men, women, children, infants who had nothing to do with the crisis” in that region. Among the dead was at least one American, Quinn Lucas Schansman, a dual United States-Dutch citizen​.

“We are going to make sure the truth is out,” Mr. Obama said.

The president called the downing of the plane an “outrageous” event and said that it should underscore for people around the world that “it is time for peace” in the region.

Mr. Obama said that the United States would “continue to lead efforts in the international community to de-escalate” the situation between Ukraine and Russia, but he did not announce any further sanctions against Russia.