Baghdad, Iraq ? Iraqi forces backed by American soldiers raided one of the country’s most important Sunni mosques as worshippers were leaving after Friday prayers — part of a crackdown on militant clerics opposed to the U.S.-led attack on Fallujah. Witnesses said at least three people were killed and 40 arrested.

Congregants at the Abu Hanifa mosque said they heard explosions inside the building, apparently from stun grenades. Later, a reporter saw a computer and books, including a Quran, scattered on the floor of the imam’s office near overturned furniture. U.S. soldiers were seen inside the mosque compound.

Insurgents said to be reeling from the loss of their base in Fallujah struck back Friday with car bombings and by firing rockets or mortars at the Green Zone, the leafy Baghdad enclave that houses the headquarters of the Iraqi and U.S. leadership here. Six people were killed in one car bombing in Baghdad, police said.

In the northern city of Mosul, where guerrillas launched an uprising last week, Iraqi forces backed by American troops raided a hospital allegedly used by insurgents, detaining three people, U.S. officials said.

The overnight raid in Mosul followed an operation late Thursday in which Iraqi military and police units killed 15 insurgents and captured 10, according to deputy Gov. Khasro Gouran. A car bomb attack Friday on a U.S. patrol in Mosul injured one American soldier, the U.S. military said.

A statement posted on an Islamic Web site in the name of Jordanian terror leader Abu Musab al-Zarqawi’s group said it had “slaughtered” two Iraqi National Guard officers “in the presence of a big crowd” in Mosul. The claim could not be verified.

But The New York Times, on its Web site Friday, quoted a 35-year-old businessman in Mosul who said he witnessed the executions. Senan Shukri told the Times that the two Iraqi soldiers were brought by car to a public square, where they were surrounded and immediately beheaded. The killers then announced that they would also decapitate anyone who removed the bodies, he said.

In Baghdad, American troops were seen securing the outer perimeter of the mosque, located in the Azamiyah district, and sealing it off before Iraqi police entered. At least 10 U.S. armored vehicles were parked at the mosque, along with two vehicles carrying about 40 Iraqi National Guardsmen, witnesses said.