U.S. President Barack Obama said Tuesday he is directing the Pentagon to carry out a thorough and unstinting investigation of the weekend killings of 16 Afghan civilians, allegedly by a U.S. soldier.

"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."

Speaking at the White House, Obama said the killings do not reflect American values or the U.S. military. But he said it won't change U.S. commitment to finishing the job in Afghanistan. Obama says the U.S. will "responsibly" draw down its forces between now and the end of 2014, the date he set with allies to close out the war.

UN Secretary General Ban Ki-Moon said he was "very sad to hear about this shocking incident."

"We expect that an investigation will rapidly establish the facts and that those responsible will be held accountable and that the public will be fully informed," he told a group of correspondents at the UN.

In Afghanistan, Taliban militants opened fire Tuesday on a delegation of senior Afghan officials — including two of President Hamid Karzai's brothers — visiting villages in southern Afghanistan where the civilians were killed.

First protests

The attack came as students in the east staged the first significant protest in response to Sunday's shootings, raising worries of a repeat of the wave of violent demonstrations that rocked the nation after last month's burning of Qur'ans by troops at a U.S. base.

The militants killed an Afghan soldier who was providing security for the delegation in Balandi village, said Gen. Abdul Razaq, the police chief for Kandahar province where the visit took place. Another Afghan soldier and a military prosecutor were wounded, he said.

The delegation was in a mosque for a memorial service for those killed Sunday when the gunfire erupted.

One of the president's brothers, Qayum Karzai, said the attack didn't seem serious to him.

"We were giving them our condolences, then we heard two very, very light shots," said Karzai. "Then we assumed that it was the national army that started to fire in the air."

He said the members of the delegation, which also included Kandahar Gov. Tooryalai Wesa and Minister of Border and Tribal Affairs Asadullah Khalid, were safe and headed back to Kandahar city.

Taliban threats

Before the attack on the delegation, the Taliban vowed to kill and behead those responsible for the civilian deaths in the two villages in Panjwai district, considered the birthplace of the militant group.

Nine of the 16 killed were children, and three were women, according to President Karzai.

A U.S. military official says probable cause has been found to keep holding the American soldier suspected in the killings.

Col. Gary Kolb, a spokesman for the U.S.-led military coalition in Kabul, says a 48-hour probable cause assessment has been completed and that the unidentified staff-sergeant continues to be confined.

U.S. Secretary of Defence Leon Panetta has said that the soldier could face capital punishment.

Villagers have described the soldier as stalking from house to house in the middle of the night, opening fire on sleeping families and then burning some of the bodies of the dead afterward.

The killings have caused outrage in Afghanistan but have not sparked the kind of violent protests seen last month after American soldiers burned Muslim holy books and other Islamic texts.

The more muted response could be a result of Afghans being used to dealing with civilian casualties in over a decade of war. Some have said the slayings in Panjwai were more in keeping with Afghans' experience of deadly night raids and airstrikes by U.S.-led forces than the Qur'an burnings were.

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Afghan protesters shout anti-U.S. slogans during a demonstration in Jalalabad province, against Sunday's shooting of at least 16 villagers by a U.S. Army staff sergeant. (Parwiz/Reuters)

But the students protesting at a university in Jalalabad city, 125 kilometres east of the capital Kabul, were incensed.

"Death to America!" and "Death to the soldier who killed our civilians!" shouted the crowd.

Soldier held in killings had sniper training

Some carried a banner that called for a public trial of the soldier, who U.S. officials have identified as a married, 38-year-old father of two who was trained as a sniper and recently suffered a head injury in Iraq.

Other protesters burned an effigy of Obama.

"The reason we are protesting is because of the killing of innocent children and other civilians by this tyrant U.S. soldier," said Sardar Wali, a university student. "We want the United Nations and the Afghan government to publicly try this guy."

Taliban spokesman Zabiullah Mujahid said in a statement sent to reporters Tuesday that the soldier should be tried as a war criminal and executed by the victims' relatives.

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An Afghan National Army soldier stands near an Afghan national flag at Forward Operating Base Joyce in Kunar province, eastern Afghanistan. Security has been boosted at bases in the country following the shooting of 16 civilians on the weekend. (Erik De Castro/Reuters)

In the aftermath of the Qur'an burnings last month, over 30 people were killed in the protests and Afghan forces turned their guns on their supposed allies, killing six U.S. troops.

The Qur'ans and other Islamic books were taken from a detention facility and dumped in a burn pit because they were believed to contain extremist messages or inscriptions. A military official said at the time that it appeared detainees were exchanging messages by making notations in the texts.

U.S.-Afghan strains appeared to be easing as recently as Friday, when the two governments signed a memorandum of understanding about the transfer of Afghan detainees to Afghan control — a key step toward an eventual strategic partnership to govern U.S. forces in the country after most combat troops leave in 2014.

In the Afghan parliament, however, lawmakers called Monday for a halt to talks on the strategic partnership document until there was confirmation the soldier behind the shooting rampage would face justice in Afghanistan.

U.S. officials have said initial reports indicate the soldier turned himself in after the shootings.

The soldier was deployed to Afghanistan on Dec. 3 with the 2nd Battalion, 3rd Infantry Regiment of the 3rd Stryker Brigade, 2nd Infantry Division, based at Joint Base Lewis-McChord located south of Seattle, according to a congressional source, speaking on the condition of anonymity because of the sensitivity of the matter.

He was sent on Feb. 1 to Belambai, the base located about a kilometre from one of the villages that was attacked, the source said.