Two U.S. troops were killed at a NATO-Afghan base on Thursday by up to two Afghan gunmen, one of them apparently a soldier and one a civilian teaching a course on the base, NATO and Afghan officials said.

A local official said both of the slain service members were American. The shootings raise to six the number of American troops killed in recent days on military bases or at Afghan government facilities.

Two of those troops were shot dead by an Afghan soldier a week ago, when a mob attacked a joint base during protests over the burning of Korans by U.S. military personnel at the Bagram air base earlier in February, an action U.S. officials have said was accidental. Two others, acting as advisers, were slain at the Afghan Interior Ministry on Saturday by a police officer, prompting NATO to withdraw its personnel from ministries.

At least 10 Americans have been killed this year by Afghan security forces or militants wearing military or police uniforms.

No motive was immediately given for Thursday’s slayings, and officials differed as to whether one or two men were responsible. The shootings came hours after NATO’s top commander in Afghanistan, Marine Gen. John R. Allen, decided to allow the return of selected coalition advisers to some Afghan government ministries.

On Thursday, a senior U.N. official in Kabul seconded a call by Afghan President Hamid Karzai for the U.S. military to discipline those involved in the Koran burning, Reuters reported. President Obama and top U.S. defense officials have apologized profusely for the incident.

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

Obama’s formal apology to Karzai for the burnings had drawn criticism from political opponents, as well as some members of the U.S. military. The president defended his decision Wednesday, saying it had “calmed things down” after the incident provoked deadly violence across the country .

“We’re not out of the woods yet,” he said in an interview with ABC News’s Bob Woodruff at the White House. “But my criteria in any decision I make, getting recommendations from folks who are actually on the ground, is what is going to best protect our folks and make sure that they can accomplish their mission.”

Thursday’s killings happened before dawn at a base in the Zhari district of southern Kandahar, said Niaz Mohammad Sarhadi, Zhari’s district chief. He said a local civilian teaching Afghan army literacy courses got control of a soldier’s rifle and fatally shot the two American service members, who were on duty inside the base.

A NATO statement said that in addition to the Afghan civilian, a second man who was believed to be a member of the Afghan National Army also was involved in the shootings. The two men “turned their weapons indiscriminately” against NATO and Afghan soldiers, the statement said, killing the two coalition troops.

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

Capt. Justin Brockhoff, a spokesman for NATO, confirmed the killing of two coalition troops but refused to identify them by nationality, citing the need to notify next of kin of their deaths. Brockhoff said he did not know whether the deaths would prompt Allen to again remove U.S. advisers from Afghan ministries.

More than 70 NATO troops are estimated to have been killed by rogue Afghan colleagues or insurgents who have managed to infiltrate in the ranks of security forces in recent years.

The killings are seen as a major challenge for the NATO coalition, which plans to withdraw all combat troops by 2014 and wants to focus on training and equipping the nascent Afghan army so it can stop the resurgent Taliban and stabilize Afghanistan.

Afghan officials say they are trying to introduce new procedures to prevent the penetration of insurgents into the ranks of the Afghan army and to identify soldiers who have sympathy for insurgents.

Staff researcher Julie Tate contributed to this report.