Four American service members were killed by an Afghan policeman early today, the third attack on coalition forces in three days, bringing the death toll in the recent violence to eight.

The attacks -- two "green on blue" incidents and an assault on a coalition base by 15 Taliban fighters -- come as tensions flared across the Muslim world over an anti-Islam film that was produced in the United States.

In the latest attack, an Afghan police officer turned his gun on NATO troops at a remote checkpoint in southern Afghanistan before dawn. Four U.S. soldiers were killed before the shooter escaped, bringing the total of coalition troops killed to six in just two days.

International Security Assistance Force officials say the attacker is still at large, and noted it's unclear if there were multiple assailants. It is still unclear exactly what transpired at the checkpoint, according to Afghan officials, who say American forces responded to an attack on the checkpoint to help the Afghan police.

"The checkpoint was attacked last night. Then the police started fighting with the Americans," Ghulam Gilani, deputy police chief of Zabul province, said. "Whether they attacked the Americans willingly, we don't know."

A Taliban spokesman said the police involved were not affiliated with the Taliban insurgency.

Also today, NATO confirmed that an airstrike that killed as many as 45 insurgents also killed five to eight Afghan civilian casualties -- including women and children.

"ISAF takes full responsibility for this tragedy," the coalition said in a statement released after the strike.

The civilian casualties can only increase tensions in the country, and villagers who drove the bodies of the dead to the privincial capital, Mehterlam chanted "Death to America," Laghman provincial government spokesman Sarhadi Zewak said.

"Protecting Afghan lives is the cornerstone of our mission and it saddens us when we learn that our action might have unintentionally harmed civilians," said Jamie Graybeal, a spokesman for international military in Afghanistan.

So far this year, 51 coalition troops have been killed by Afghan service members. At least 12 such attacks happened in August alone, leaving 15 dead.  Read more at ABC News