President Barack Obama sought Tuesday to alleviate outrage over Sunday's massacre of 16 Afghan civilians by an American soldier, denouncing the killings and vowing a full investigation.

We're heartbroken over the loss of innocent life. The killing of innocent civilians is outrageous and it's unacceptable, Obama told reporters at the White House. It's not who we are as a country and it does not represent our military, and for those reasons I have directed the Pentagon to make sure we spare no effort in conducting a full investigation.

The American soldier is currently being held in military custody in Kandahar province and remains unidentified by name, but is said to be an Army sergeant from Joint Base Lewis McChord in Washington state, according to the New York Daily News and other media. The father of two could face the death penalty if convicted by a military tribunal.

The sergeant allegedly broke into the homes of villagers in the middle of the night, indiscriminately shooting civilians in their beds as they slept. Nine children were killed among the 16 victims. According to the Daily News, five people survived the shootings.

We will make sure that anybody who was involved is held fully accountable with the full force of the law, the president said. The U.S. takes this as seriously as if it were our own citizens and our own children who were murdered.

The local investigation, however, has already encountered impediments. Afghan investigators, including two of Afghan President Hamid Karzai's brothers, were shot in a firefight with insurgents while visiting the site of the massacre in the Panjawi district, a Taliban stronghold, resulting in the death of an Afghan soldier, Reuters reported.

Anti-American sentiment in Afghanistan has increased in recent weeks, following the burnings of copies of the Koran by U.S. soldiers at a NATO airbase and the latest massacre.

Thousands of Afghans protested the killings Tuesday morning, taking to the streets of eastern Afghanistan. An effigy of Obama was burned as he was branded a killer by some, with calls for him to be tried in Afghanistan, MSNBC reported.

After more than a decade of war, the Obama administration is in the process of drawing down American troops in Afghanistan, projected to be completed by the end of 2014.

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 Tuesday.

Watch video of the press conference here: