Baghdad, Iraq ? Iraqi insurgents detonated a car bomb and then hammered a military headquarters in the city of Samarra with a mortar barrage Thursday, leveling the building and killing five U.S. soldiers and one Iraqi guardsman, the U.S. military said.

American troops — backed by attack helicopters — then fanned out through the city to hunt down the attackers in clashes that lasted into the late afternoon. Tanks deployed in the streets; smoke rose above a mosque.

The violence also killed three civilians, medical officials said. As many as 44 people were wounded, including 20 American soldiers and four Iraqi guardsmen, the military and hospital officials said.

Also Thursday, the Philippines prohibited its citizens from traveling to Iraq to work after militants released a videotape threatening to kill a Filipino hostage if the country did not withdraw its troops.

Early Friday, the Al-Jazeera television network broadcast a video showing Abu Musab al-Zarqawi’s Tawhid and Jihad group threatening to execute two Bulgarian hostages if the U.S. military did not release all Iraqi detainees within 24 hours.

The video showed the two unidentified Bulgarians sitting with their hands cuffed, flanked by three masked, armed men. The group had previously claimed responsibility for the beheading of U.S. businessman Nicholas Berg and South Korean translator Kim Sun-il.