Two American soldiers were shot dead Thursday at a NATO military base in southern Afghanistan when two men, one of whom was believed to be an Afghan soldier, turned their weapons against them. The shootings bring to six the number of U.S. service members killed in recent days on bases or at Afghan government facilities. 

The motive for Thursday's predawn attack at the base in the Zhari district of Kandahar wasn't provided by NATO, U.S. or Afghan officials. 

One of the gunmen was wearing civilian clothing and the other was believed to be a member of Afghanistan's army, according to a statement from the NATO-led International Security Assistance Force, or ISAF.

Two individuals, one believed to be an Afghan National Army service member and the other in civilian clothing, turned their weapons indiscriminately against International Security Assistance Force and Afghan National Security Force service members in southern Afghanistan today, killing two ISAF service members, the statement said.

The civilian involved in the shooting teaches a course on the base, the Washington Post cited NATO and Afghan officials as saying.

A senior defense official told NBC News that both of the NATO soldiers were American. Their nationality was confirmed to CBS News by an Afghan official, while the Post cited a local official for the confirmation.

Officials differed as to whether one or two men were responsible for Thursday's violence, according to the Washingon Post. At least two of the killings from last week were said to be retribution for the  burning of Qurans by U.S. military personnel at the Bagram air base earlier in February.

The latest violence came hours after NATO's top commander in Afghanistan, U.S. Marine Gen. John Allen, decided to allow the return of selected coalition advisers to some Afghan government ministries, the Post noted. They had been withdrawn last week because of the killing of the four American service members

Two of those service members were shot dead by an Afghan soldier a week ago, when a mob attacked a joint base during protests over the Quran burning, which U.S. officials have said was accidental. The other two, acting as advisers, were slain at the Afghan Interior Ministry last Saturday by a police officer, prompting NATO to withdraw its personnel from ministries.

On Thursday, a senior United Nations official in Afghanistan's capital, Kabul, seconded a call by Afghan President Hamid Karzai for the U.S. military to discipline those involved in the Quran-burning, Reuters reported. President Barack Obama and top U.S. defense officials have apologized for the incident.

After the first step of a profound apology, there must be a second step ... of disciplinary action, Jan Kubis, special representative in Afghanistan for the U.N. secretary general, told a news conference, according to Reuters. He said the United Nations rejected and condemned the burnings. It doesn't matter that it was a mistake.

Niaz Mohammad Sarhadi, district chief in Zhari, where the base involved in Thursday's shootings is located, told the Washington Post that a local civilian teaching Afghan army literacy courses got control of a soldier's rifle and fatally shot the two Americans, who were on duty inside the base. 

A third U.S. soldier was wounded in the attack, Sarhadi said. NATO troops responded by opening fire themselves, he claimed, killing both the the teacher and an Afghan soldier.

A NATO spokesman, Capt. Justin Brockhoff, confirmed for the Post the killing of two coalition members but refused to identify them by nationality, citing the need to notify relatives of their deaths. Brockhoff, according to the newspaper, said he didn't know if the deaths would prompt Allen to again remove U.S. advisers from Afghan ministries.