WASHINGTON — The alleged massacre of Afghan civilians by a U.S. soldier is “outrageous and unacceptable,” President Barack Obama said today, and he promised a thorough and unstinting Pentagon investigation.

“The United States takes this as seriously as if it was our own citizens, and our children, who were murdered. We’re heartbroken over the loss of innocent life,” Obama said. He sounded stern and emotional in brief remarks on the weekend killings made before an unrelated White House event.

“I’ve directed the Pentagon to make sure that we spare no effort in conducting a full investigation,” Obama said. “We will follow the facts wherever they lead us and we will make sure that anybody who is involved is held fully accountable with the full force of the law.

Defense Secretary Leon Panetta has said the death penalty could be possible in the case.

Obama’s message was aimed at Afghans and at Americans for whom the killings were a reminder that tens of thousands of U.S. forces are fighting in Afghanistan more than 10 years after the war began.

Obama insisted that the killings will not change U.S. commitment to finishing the job in Afghanistan, but he was clearly trying to reassure Americans that he will close out the war.

“Make no mistake, we have a strategy that will allow us to responsibly wind down this war,” Obama said.

“We’re steadily transitioning to the Afghans who are moving into the lead. And that’s going to allow us to bring our troops home.”

He repeated the timetable for bringing forces home that he had already laid out: 23,000 troops by the end of this summer, on top of 10,000 removed last year. He did not give a schedule for withdrawal of the approximately 68,000 U.S. forces that will remain in Afghanistan at the end of this year.

The U.S. and NATO allies agreed more than a year ago to leave forces in Afghanistan through 2014. There is political pressure in Europe, and increasingly in the United States, to speed up that deadline.

“There’s no question that we face a difficult challenge in Afghanistan, but I am confident that we can continue the work of meeting our objectives, protecting our country and responsibly bringing this war to a close,” Obama said.