KABUL - A gunman has killed two US military advisers with shots to the back of the head inside a heavily guarded ministry building on Saturday, and NATO ordered military workers out of Afghan ministries as protests continued over the burning of copies of the Quran at a US army base.

The Taliban claimed responsibility for Saturday's Interior Ministry attack, saying it was retaliation for the Quran burnings, after the US servicemen - a lieutenant colonel and a major - were found dead on the floor of an office that only people who know a numerical combination can get into, Afghan and Western officials said.

The top commander of US and NATO forces recalled all international military personnel from the ministries, an unprecedented action in the decade-long war that highlights the growing friction between Afghans and their foreign partners at a critical juncture in the war.

The US-led coalition is trying to strengthen Afghan security forces so they can lead the fight against the Taliban and foreign troops can go home. That mission, however, requires a measure of trust at a time when anti-Western sentiment is at an all-time high.

Afghan Defense Minister General Abdul Rahim Wardak called US Defense Secretary Leon Panetta to apologize for the shooting and offer his condolences, Pentagon press secretary George Little said in a statement released in Washington.

"This act is unacceptable and the United States condemns it in the strongest possible terms," Little said.

Security is tight in the capital, which is covered in snow, and foreigners working at the US embassy and at international organizations have been banned from leaving their compounds.

US officials said they were searching for the assailant, who has not been identified by name or nationality.

The two US service members were found by another foreigner who went into the room, according to the Afghan official, who spoke on condition of anonymity because he was not authorized to disclose details about the shootings. 

Authorities were poring over security camera video for clues, the Afghan official said.

Taliban spokesman Zabiullah Mujahid identified the shooter as one of its sympathizers, Abdul Rahman. He said an accomplice inside the ministry helped Rahman get inside the compound to kill the two US military advisers in retaliation for the Quran burnings.

"After the attack, Rahman informed us by telephone that he was able to kill four high-ranking American advisers," Mujahid said. The Taliban often inflate death tolls and sometimes claim responsibility for killings they did not conduct.

Little, the Pentagon press secretary, said Wardak indicated that President Hamid Karzai was assembling religious leaders and other senior Afghan officials to take urgent steps to protect coalition forces.

Afghan President Hamid Karzai called for calm on Sunday in a televised news conference after at least 29 people died in five days of demonstrations over the Quran burnings.

Karzai "condemned with the strongest words" the treatment of Islam's holy book and said the perpetrators should be punished, but told his countrymen: "Now that we have shown our feelings it is time to be calm and peaceful."

He said he respected the emotions of Afghans upset by the Quran burnings, but urged them not to let "the enemies of Afghanistan misuse their feelings".

Seven US troops were wounded on Sunday when demonstrators protesting the Quran burning at a US base threw a grenade into their base in the northern province of Kunduz, police said.

"The demonstrators hurled a hand grenade at US special forces base in Imam Sahib town of Kunduz province -as a result seven US special forces were wounded," Kunduz police spokesman Sayed Sarwar Hussaini said.

A spokesman for US-led NATO forces in Afghanistan said: "According to initial reports, an explosion occurred outside of an ISAF installation in northern Afghanistan early this afternoon."

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