KABUL, Afghanistan –  At least one Afghan police officer turned his gun on NATO troops at a remote checkpoint in southern Afghanistan before dawn Sunday, killing four American service members before escaping, according to Afghan and international officials.

It was the third attack by Afghan forces or insurgents disguised in military uniforms against international forces in as many days, killing eight troops in all.

Recent months have seen a string of such insider attacks by Afghan forces against their international counterparts. The killings have imperiled the military partnership between Kabul and NATO, a working relationship that is key to the handover of security responsibilities to Afghan forces as international troops draw down.

The most recent attacks also come amid an international uproar about an Internet video mocking the Prophet Muhammad that many fear could further aggravate Afghan-U.S. relations. The video has sparked protests throughout the Muslim world and the Afghan government blocked the YouTube site that hosts the video and its parent company, Google Inc., over the weekend in a move to prevent violent protests.

So far, protests in Afghanistan have remained peaceful.

Details of Sunday's attack were slow to come out because it took place in a remote area, said Jamie Graybeal, a spokesman for the international military in Afghanistan. He said the attacker was still at large but it was unclear if there were multiple assailants.

"The attack took place in the vicinity of an outpost in southern Afghanistan. It is my understanding that it was a checkpoint," Graybeal said. International forces often work with Afghan police to man checkpoints as part of the effort to train and mentor the Afghan forces so that they can eventually operate on their own. The goal is to turn over all security responsibility for the country to the Afghans by the end of 2014, though numbers of NATO forces have already been reduced in many areas.

Afghan officials said the checkpoint in Zabul province's Mizan district first came under attack from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.

It was not clear if some of the Afghan police turned on their American helpers in the middle of the battle with the insurgents, or afterward, or were somehow forced into attacking the American troops by the insurgents, Gilani said.

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

Taliban spokesman Qari Yousef Ahmadi said the police were not affiliated with the Taliban insurgency.

"But they are Afghans and they know that Americans are our enemy," Ahmadi told The Associated Press.

He said all four of the dead were American. A U.S. official speaking on anonymity because the information had not been officially released confirmed that the four killed were American.

The coalition said in a statement that they were investigating what happened.

There were also international troops wounded, Graybeal said. He said that early reports showed two were hurt and that they were receiving treatment. He did not say how serious the injuries were.

So far this year, 51 international service members have died at the hands of Afghan soldiers or policemen or insurgents wearing their uniforms. At least 12 such attacks came in August alone, leaving 15 dead.

On Saturday, a gunman in the uniform of a government-backed militia force shot dead two British soldiers in Helmand district in the southwest.

Britain's defense minister said the two soldiers, from 3rd Battalion the Yorkshire Regiment, were killed at a checkpoint shooting in Nahri Sarraj district of Helmand province in southern Afghanistan, where the Taliban have their strongest roots. NATO said earlier that the gunman was wearing a uniform used by the Afghan Local Police, a village-level fighting force overseen by the central government.

That strike came a day after insurgents wearing U.S. Army uniforms attacked a military base, killing two American Marines, wounding nine other people and destroying six Harrier fighter jets, military officials said. Fourteen insurgents were killed. The Taliban claimed responsibility for that attack and said that it was revenge for the video insulting Prophet Muhammad.

In the capital on Sunday, several hundred university students chanted "Death to America!" and "Long life to Islam!" over several hours to protest the video. Riot police cordoned off the area and the protest ended without incident in the early afternoon. A smaller protest went forward in the western city of Herat.