KABUL, Afghanistan — Another two United States soldiers were killed in Afghanistan on Thursday when an Afghan soldier in league with a civilian Afghan literacy instructor killed a tower guard and attacked with gunfire and a rocket the barracks where American soldiers were sleeping at a joint base in the south of the country, officials said.

The killings follow the shooting deaths of four American soldiers last week, including two at the Afghan Interior Ministry, after the outbreak of nationwide protests in reaction to the burning of Korans at a NATO base in Afghanistan.

Four other Americans were injured in the attack on Thursday, which took place in pre-dawn hours around 2.30 a.m. local time at a joint Afghan-NATO base in Kandahar Province in southern Afghanistan.

The two shooters fled but American forces called in air support, and the two men were killed by helicopter gunfire more than a mile from the base, said Masoom Khan, the chief of police of the Zhari district in Kandahar Province where the attack took place.

The base in Kandahar Province is close to the village of Sangsar, the hometown of the Taliban leader Mullah Muhammad Omar and the birthplace of the group. Mr. Khan said the shooters were suspected of having links with the insurgency.

He said the Afghan Army soldier was a platoon leader and had taken part in joint patrols with the Americans.

“We believe that both of the attackers had links with the Taliban,” he said.

In a statement, NATO would only confirm that two coalition service members had been killed in the attack.

“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 read.

Two earlier American deaths — in the Afghan Interior Ministry on Saturday — prompted NATO to immediately withdraw hundreds of military advisers and trainers from government ministries in Kabul. On Thursday, a NATO spokesman, Brig. Gen. Lewis Boone, said some advisers were returning to ministries, but he did not specify which offices were involved.

Three investigations are underway into the Koran burning at Bagram Air Base last week. Days of deadly protests ensued, claiming at least 30 Afghan lives in addition to the American solders.

One investigation is by Americans, one is by Afghans and one is a joint Afghan-American inquiry. The formal American military investigation is the only one that can lead to punishment, while the other two will include recommendations but do not carry formal legal weight
.