BAGHDAD, Iraq - Insurgents ambushed a convoy of American and Iraqi forces in the northern city of Mosul, detonating a roadside bomb and firing from a mosque in an attack that killed three Iraqi National Guardsmen, the U.S. military said today.

In Baghdad, U.S. forces detained six suspects in the Jan. 4 slaying of the governor of Baghdad province, Ali al-Haidari, the military said today. The suspects were detained Tuesday during a raid on a house in northern Hurriyah.

Brig. Gen. Jeffery Hammond, assistant commander of the 1st Cavalry Division, which controls Baghdad, said he believed two of those detained were directly involved in the slaying.

Al-Haidari was the highest-ranking Iraqi official killed since the former president of the now defunct Governing Council, Abdel-Zahraa Othman, was assassinated in May.

In Mosul, the troops were bringing heaters and other supplies to a school when they came under attack on Tuesday, a military statement said. The convoy was first hit with a roadside bomb and then sprayed with gunfire from a nearby mosque. Three Iraqi troops were killed and six were wounded. No Americans were reported hurt.

Violence has surged in the run-up to Iraq's Jan. 30 election, and Mosul has been a major trouble spot in recent weeks. On Tuesday, Prime Minister Ayad Allawi acknowledged that parts of Iraq probably won't be safe enough for people to vote and said he plans to boost the size of the country's army from 100,000 to 150,000 men by year's end.

Allawi discussed preparations for this month's election by telephone with President Bush on Tuesday, and both leaders underscored the importance of going ahead with the vote as planned.

The prime minister said at a news conference that "hostile forces are trying to hamper this event."

Also Tuesday, gunmen stopped three trucks carrying new Iraqi coins south of Baghdad and killed the drivers, stole the money and set the trucks on fire, a police official said.

The attack occurred about 12 miles southeast of Baghdad. The trucks were carrying the money from the southern port city of Basra to the Central Bank of Iraq in Baghdad, the official said on condition of anonymity.

Meanwhile, a U.S. soldier was killed in action in Iraq's volatile western Anbar province, a military statement said today. The statement said only that the soldier, assigned to the 1st Marine Expeditionary Force, was killed Tuesday. The unit is based at Camp Fallujah west of Baghdad.

The death brought to 1,356 the number of American troops killed in Iraq since the invasion in March 2003.