President Barack Obama said on Monday the massacre of 16 villagers by a US soldier increases his determination to get American troops out of Afghanistan, while a US official said the accused staff sergeant previously had suffered traumatic brain injury.

Sunday's shootings triggered angry calls from Afghans for an immediate American exit. Obsama said there should not be a "rush to the exits" for US forces who have been fighting in Afghanistan since 2001 and that the drawdown must be carried out in a responsible way.

The accused US Army staff sergeant walked off his base in the southern province of Kandahar in the middle of night and gunned down at least 16 villagers, mostly women and children.

Defense Secretary Leon Panetta said the death penalty could be sought in the US military justice system against the soldier, whose name has not been publicly disclosed.

Referring to Sunday's massacre, Obama said in an interview with KDKA, a CBS affiliate in Pittsburgh: "It makes me more determined to make sure we're getting our troops home."

"It's time. It's been a decade, and, frankly, now that we've gotten (Osama) bin Laden, now that we've weakened Al-Qaeda, we're in a stronger position to transition than we would have been two or three years ago," Obama added, referring to the Al-Qaeda leader killed by US forces last year in Pakistan.

The Army staff sergeant accused in the incident was treated for traumatic brain injury suffered in a vehicle rollover in 2010 during a previous deployment in Iraq, a US official said. The official, speaking on condition of anonymity, said it was premature to state whether there was any link between the brain injury and Sunday's shootings.

Panetta portrayed the shooting as an isolated event that would not alter plans for a gradual, orderly withdrawal of American combat forces by the end of 2014.

"War is hell. These kinds of events and incidents are going to take place, they've taken place in any war. They're terrible events. And this is not the first of those events, and it probably won't be the last," the US defense secretary told reporters on a flight to Kyrgyzstan.

"But we cannot allow these events to undermine our strategy or the mission that we're involved in."

Obama was pressed in another separate interview with WFTV, an ABC affiliate in Orlando, on whether there were parallels between the killing of 16 Afghan villagers and the notorious 1968 My Lai massacre of the Vietnam War.

"It's not comparable," Obama said.

"It appeared you had a lone gunman who acted on his own," he said of the Afghanistan incident. "In no way is this representative of the enormous sacrifices that our men and women have made in Afghanistan."

Panetta said the goal was to try the case within the US military justice system. Asked whether the death penalty could be considered, Panetta replied: "My understanding is that in these instances that could be a consideration."

Obama and America's NATO partners intend to pull most of their troops out of Afghanistan by the end of 2014, leaving an inexperienced local army in charge. Officials in Washington denied that the killings might alter that schedule.

"I do not believe this incident will change the timetable of a strategy that was designed and is being implemented in a way to allow for the withdrawal of US forces, to allow for their transfer of lead security authority over to the Afghans," White House spokesman Jay Carney told reporters.

Just days before Sunday's attack, Kabul and Washington had made significant progress in negotiations on a strategic partnership agreement that would allow American advisers and special forces to stay in Afghanistan after most foreign combat troops leave at the end of 2014.

But securing a full deal may be far more difficult now. "This could delay the signing of the Strategic Partnership Agreement," an Afghan government official told Reuters.

The attack was the latest incident to ignite Afghan anger at the United States, coming on the heels of US soldiers' burning of copies of the Koran on a NATO base last month, and other incidents that have intensified America's perception problem in Afghanistan.

Panetta, answering questions for the first time about the shooting rampage, said US officials were still uncertain about the motives behind it.

The accused soldier was part of the 2-3 Infantry, 3rd Stryker Brigade Combat Team, from the Lewis-McChord base in Washington state. After serving three tours in Iraq, he arrived in Afghanistan in December and has been at the Belambai base since 1 February.

Based on preliminary findings of a US investigation into the incident, Panetta gave this account of the soldier's actions: "He went out in the early morning and went to these homes and fired on these families. And then at some point after that, came back to the forward operating base and basically turned himself in, told individuals what had happened."

US officials, who have rushed to distance the shootings from the efforts of the 90,000-strong US force that over the past year has beaten the Taliban back from much of southern Afghanistan, say an investigation was under way but did not know when it would conclude.

Afghanistan's parliament condemned the killings, saying Afghans had run out of patience with the actions of foreign forces and the lack of oversight. Civilian deaths have long been one of the main sources of tension between Kabul and Washington.