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Afghan officials said the checkpoint in Zabul province’s Mizan district came under attack first 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, Mr. 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,” Mr. Gilani said.

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.

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

“But they are Afghans,, and they know that Americans are our enemy,” Mr. Ahmadi told the Associated Press. In an emailed statement, he said the police who fled have joined up with the insurgency.

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

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.

Mirwais Khan reported from Kandahar, Afghanistan. Associated Press writer Rahim Faiez contributed to this report from Kabul.