BAGHDAD, Iraq - Insurgents fired mortar rounds today at a headquarters used by U.S. and Iraqi forces in the city of Samarra, destroying the building and killing four American soldiers and one Iraqi guardsman, the military said.

A fifth U.S. soldier was unaccounted for and 20 others were wounded in the attack, said Maj. Neal O'Brien, the spokesman for the 1st Infantry Division. U.S. troops secured the area around the collapsed building.

Also today, the Philippines barred its contract workers from traveling to Iraq after militants released a videotape threatening to kill a Filipino hostage if the country does not withdraw its troops.

Samarra was wracked by clashes throughout the day that killed three civilians and injured 20, said Dr. Abid Tawfiq, director of the Samarra General Hospital.

Earlier in the day, a U.S. military convoy in Samarra was targeted by a roadside bomb that wounded one U.S. soldier, O'Brien said.

About 25 minutes after the mortar attack - when radar determined where it had originated - U.S. soldiers responded with four 120 mm mortars.

In Baghdad, meanwhile, an explosion ripped through a car outside a textile factory in the Dora neighborhood, killing a former senior Baath Party official, said police Lt. Anmar Yassin. Authorities didn't know the cause of the explosion that killed Ali Abbas Hassan.

Today's violence came a day after Iraq unveiled emergency laws giving the government broad powers to fight its enduring insurgency.

The Philippines, with only 51 troops in Iraq, make up a tiny fraction of the Multinational Force. But the more than 4,000 Filipino civilians work as contractors for the U.S. military, serving food, cleaning toilets and forming the backbone of the support staff for U.S. troops.

In addition, many with specialized training provide security at important facilities, construct buildings and furniture and maintain roads. The U.S. military, which has diverted as many soldiers to combat duty as possible, would be hard pressed to operate in Iraq without the extra manpower the Filipinos provide.

The security laws unveiled Wednesday allow Prime Minister Iyad Allawi to set curfews, impose limited martial law, send security forces on searches and freeze suspects' assets and monitor their communications. He can also assign military leaders to run restive areas.

Allawi, a secular Shiite with close CIA links, can only invoke his new powers with the unanimous approval of the Presidential Council made up of the president, who is a Sunni Arab, and two vice presidents, a Kurd and a Shiite.

Secretary of Defense Donald H. Rumsfeld backed the new laws. "In terms of if they will be more effective, I would think so," he said, but he would not say if U.S. troops would help enforce the security laws.