The assault on the outpost, which U.S. soldiers finally repulsed, appeared to be better coordinated than past attacks, with guerrillas apparently pulling out their strongest assaults in an effort to derail Jan. 30 elections, U.S. military spokesman Lt. Col. Paul Hastings said.

"The terrorists are growing more desperate in their attempts to derail the elections and they're trying to put it all on the line and give it all they can," Hastings said.

Mosul, Iraq's third-largest city, has become a hotbed of insurgent activity in the past several months. A suicide bomber infiltrated a U.S. base near the city last week, detonating his explosives in a dining tent and killing 22 people, including 18 Americans. The radical Ansar al-Sunnah Army claimed responsibility for the attack.

Wednesday' clashes began when a truck approached the base and American troops opened fire. The truck, laden with 1,000 pounds of explosives, blew up just outside, Hastings said.

Reinforcements came under fire by guerrillas using automatic weapons and rocket-propelled grenades and moving in squads of between 10 and 12. A heavily armored Stryker vehicle that had left the outpost moments before the truck bomb came across seven roadside bombs that had been laid out for its return, Hastings said. The bombs were detonated safely.

The Americans then called in strikes by F-18 and F-16 fighter jets, which launched three Maverick missiles and conducted several strafing runs against the insurgents. The result was 25 insurgents and one American soldier killed. Twenty Americans were wounded, but 17 returned to duty within hours.

Insurgents have shown an increasing sophistication in their attacks. In Baghdad on Wednesday, guerrillas used an anonymous tip to lure Iraqi police and national guards to a house in a staunchly Baathist neighborhood. They then set off a massive explosion in the house, killing 22 civilians and seven officers.

The latest clash in Mosul came as U.S. troops launched a new offensive in an area south of the capital dubbed the "triangle of death," in an apparent effort to secure the region ahead of the crucial parliamentary election on Jan. 30.

Brig. Gen. Jeffery Hammond, assistant commander of the 1st Cavalry Division that controls Baghdad, said Wednesday that U.S. troops were focusing on areas around Mahmoudiya, a town about 25 miles south of the capital.

U.S. and Iraqi forces have come under repeated attacks by car bombs, rockets, and small arms fire in the area. The latest operation followed a weeklong campaign in November and early December to root out insurgents in the same region.

The insurgent group that claimed responsibility for the Dec. 21 suicide bombing of the Mosul base warned Iraqis not to take part in the elections.

"We also warn everyone to keep away from all military targets, whether they were bases, American Zionist patrols, or the forces of the pagan guard and police," Ansar al-Sunnah said in a statement released Wednesday.

In another statement on its Web site Thursday, Ansar al-Sunnah and two other militant groups denounced democracy as un-Islamic. The statement said that democracy could lead to passing un-Islamic laws, such as permitting homosexual marriage, if the majority agrees to it.

"Democracy is a Greek word meaning the rule of the people, which means that the people do what they see fit," the statement said. "This concept is considered apostasy and defies the belief in one God - Muslims' doctrine."

The warning followed Monday's audiotape statement from al-Qaida leader Osama bin Laden urging Iraqis to boycott the elections and praising attacks against Americans and those who cooperate with them.

President Bush denounced bin Laden's appeal, saying the election marks a crossroads for Iraq.

Insurgents have intensified their strikes against the security forces of Iraq's U.S.-installed interim government as part of a continuing campaign to disrupt the elections for a constitutional assembly.

Government troops are supposed to protect polling stations, and the insurgents' strategy - which includes attacking police stations, checkpoints and patrols - appears aimed at demonstrating the security forces are incapable of handling the job.