For the third day in a row, an Afghan "ally" turned against NATO troops before dawn Sunday, killing four American service members, Afghan and international officials said.

Eight coalition troops have been killed by Afghan forces or insurgents disguised in military uniforms in three days now, the Associated Press reports. Recent months have seen a string of such insider "green-on-blue" attacks by Afghan forces against their international counterparts. 

One of the six members of the Afghan National Police operating the observation post with six coalition troops was also found dead, while the other five had disappeared, Reuters reported.

"The fighting had stopped by the time the responders arrived," said Maj. Adam Wojack, a spokesman for the NATO-led coalition.

Sunday's shooting took place in Zabol, a province where U.S. forces are based, according to a local official, who said the four soldiers killed were American.

Meanwhile, according to Afghan officials, airstrikes by NATO planes killed eight women and girls in a remote part of the country, fueling a long-standing Afghan grievance over civilian casualties.

Villagers from a remote part of Laghman province's Alingar district drove the bodies to the provincial capital, saying they were killed by NATO aircraft while they were out gathering firewood before dawn.

"They were shouting 'Death to America!' They were condemning the attack," said Laghman provincial government spokesman Sarhadi Zewak.

Seven injured females were also taken to hospitals, some of them as young as 10 years old, said provincial health director Latif Qayumi.

NATO forces at first said that about 45 insurgents and no civilians were killed in the attack but spokesman Jamie Graybeal stressed later that they took the charge of civilian deaths seriously and were investigating the allegations.

Afghan President Hamid Karzai condemned the airstrike and said a government investigation had been opened.

Details of Sunday's attack were sketchy because it took place in a remote area, said Graybeal.

"The attack took place in the vicinity of an outpost in southern Afghanistan. It is my understanding that it was a checkpoint."

Graybeal said one police officer was killed in the clash with NATO troops but that the other officers at the site fled and it was unclear if they were involved in the attack or not. Two international troops were wounded.

The attack came a day after two British soldiers were shot dead by an Afghan policeman while returning from a patrol in southern Helmand province.

Adding to the toll of coalition deaths caused by insider attacks over the weekend, two were killed and nine wounded in Friday's attack on Camp Bastion, one of the worst attacks on a NATO-operated base all year.

Six Harrier jets were destroyed and two were significantly damaged in the raid on the camp airfield, carried out by 15 insurgents wearing U.S. Army uniforms, the NATO-led coalition said in a separate statement on Sunday.

Operating in three teams, they succeeded in breaching the perimeter of the heavily fortified base. Britain's Prince Harry was at Camp Bastion at the time of Friday's attack, but was unharmed.

In a separate incident on Sunday, NATO-led forces arrested a Taliban fighter responsible for killing two U.S. troops when they were downed in their Kiowa helicopter in eastern Afghanistan, the coalition said in another statement.