(CNN) -- A gunman in an Afghan National Army uniform and another man shot dead two NATO soldiers at a combat outpost in southern Afghanistan Thursday, authorities said.
The dead soldiers were Americans, according to Niaz Mohammad Sarhadi, the district chief in Kandahar province, where the shooting happened. The NATO-led International Security Assistance Force, however, did not immediately specify the service members' nationalities.
Thursday's shooting was the third at a base or government building since news emerged that U.S. troops mistakenly burned Qurans and other religious materials early last week -- an incident that has sparked outrage, protests, and violence across Afghanistan.
All three shootings were carried out by men in official clothing.
Four Americans were killed in the earlier attacks. If the troops killed Thursday are indeed American, that would bring the death toll from the three attacks to six.
A top U.S. lawmaker last month cited at least 42 attacks by Afghan security forces on their international allies in the past five years.
Rep. Buck McKeon, who chairs the Armed Services Committee, said 39 of the attacks were by members of the Afghan National Security Force and three were by contractors.
"Though less than 1% of Afghan forces and security guards have attacked coalition forces, this is 42 attacks too many and the new process must do better," he said at a congressional hearing.
Most attackers were acting on their own, rather than being puppets of insurgent groups, defense officials testified.
France suspended its operations in Afghanistan after four French soldiers were killed by Afghan counterparts in eastern Kapisa province in January. That shooting came after another Afghan soldier killed two French soldiers in December.
"The French army is not in Afghanistan to be shot at by Afghan soldiers," French President Nicolas Sarkozy said after the shootings.
Coalition forces responding to Thursday's incident killed the two gunmen, said Capt. Justin Brockhoff, an ISAF spokesman.
Sarhadi said one gunman was an Afghan National Army soldier and the other, dressed in civilian clothes, was a teacher who formerly ran literacy courses for Afghan army soldiers.
Afghan civilians are sometimes used to teach Afghan soldiers English to improve their ability to work alongside ISAF.
It appeared that the teacher had been planning the attack for at least a year, Sarhadi said, because officials found pictures and other documents related to the Taliban among his possessions.
U.S. officials have called the Quran burning an error by troops who were inadvertently given the Qurans and other religious materials to dispose of because they were thought to contain extremist inscriptions. U.S. President Barack Obama has apologized for the burning.
Still, gunmen have targeted U.S. troops in the days following the burning. A man wearing an Afghan National Army uniform killed two American soldiers at a base in eastern Nangarhar province early last week.
Over the weekend, two senior U.S. officers were gunned down inside the heavily secure Afghan Interior Ministry when a junior intelligence officer turned his gun on them. The gunman, who had been fired by the ministry but recently rejoined as a driver, is still on the loose.
The incident prompted the United States to pull military advisers and embedded civilians from Afghan ministries, and France announced it was temporarily withdrawing all of its public officials in Afghan institutions to ensure their safety.
The unrest following the Quran burning has left at least 41 people dead, including the two deaths Thursday, and hundreds more wounded.
At least nine people were killed and 12 wounded in an explosion Monday near the front gate of the ISAF base at Jalalabad airport in eastern Afghanistan. There were no NATO casualties in the bombing.
The Taliban insurgency said the attack was in retaliation for the Quran burning.
In northern Kunduz province over the weekend, protesters attacked a police chief's office and a U.S. military base, Afghan and U.S. authorities said. Some threw hand grenades at the base, known as Combat Outpost Fortitude, and the resulting blasts wounded seven U.S. personnel believed to be Special Forces members, they said.
Demonstrations outside the United Nations office in Kunduz on Saturday left four civilians dead and prompted the U.N. mission there to say Monday that it is temporarily relocating its international staff.
U.S. Secretary of State Hillary Clinton said Sunday the unrest and targeting of Americans has gotten "out of hand" and needs to stop.
The Obama administration feels the Afghans have not done enough to stem the violence, according to a senior U.S. official who has access to the latest intelligence and is involved in administration discussions, but declined to be identified because of the sensitive nature of the situation.
"We are not going to settle for what has happened to our troops in recent days," the official said.
The U.S. ambassador to Afghanistan, Ryan Crocker, maintained Sunday that the Afghan security forces have given their lives to control the demonstrations and protect U.S. installations.
Crocker said the Afghans "are very much in this fight trying to protect us."
After the Interior Ministry shooting, Lt. Col. Jimmie Cummings, an ISAF spokesman, said the United States would not let the incident divide the coalition.
CNN's Masoud Popalzai contributed to this report.