Insurgents also opened fire on a bus carrying Iraqi women home from work at a military base west of Baghdad, killing four women and wounding six others. All the attacks were in the volatile "Sunni triangle" region around Baghdad.

The latest upsurge of violence came amid US talk of handing over political power to an Iraqi administration later this year and a dispute over whether this should be done before or after elections are held.

South of the capital near Diwaniya, a Spanish Civil Guard police commander was shot in the head and seriously wounded during a joint operation with Iraqi police against "members of a terrorist group", the Spanish Defence Ministry said.

Police near Falluja, a hotbed of resistance 50 km (30 miles) west of Baghdad, said guerrillas in a passing car lobbed a grenade and opened fire with AK-47 assault rifles at a checkpoint on the highway to the town of Ramadi.

"We were standing at our checkpoint and saw some cars come by. From one of them, a grenade was thrown and Kalashnikovs were fired at us," policeman Maher Mohammad said. He said the attackers wore checkered headdresses around their faces.

Two policemen and a civilian were killed in the attack, and five police were wounded. A pool of blood lay on the side of the highway, along with a police vehicle pockmarked by bullets.

A US military spokeswoman in Saddam Hussein's hometown of Tikrit said mortars and rockets had been fired at an American base near Baquba, 65 km north of Baghdad, on Wednesday evening.

"Last evening, we had a mortar attack on a forward operating base near Baquba which killed two soldiers from the 4th Infantry Division and critically wounded another," Major Josslyn Aberle said, adding that rockets were also used in the attack.

The deaths brought to 349 the number of US soldiers killed in action in Iraq since Washington launched a war in March to topple Saddam. Including non-combat deaths, the toll is 505.

"We were able to identify where the attack came from and responded by firing artillery shells at the location," Aberle said. "There are no indications, however, that any insurgents were killed in our assault."

Guerrillas fighting the US-led occupation have often targeted Iraqi police and others cooperating with foreign soldiers and international organisations. Police and hospital officials in Falluja said insurgents had attacked a bus carrying women home from work at a US base near Habbaniya on Wednesday.

Police said the Iraqi women were Christians who lived in Baghdad and were taken every day by minibus to the base west of the capital, where they worked as cleaners and cooks. Buses carrying the women had been shot at before, they said.

Spain's Defence Ministry said the wounded Civil Guard police commander, Gonzalo Perez Garcia, was head of security for a Spanish military brigade in Iraq.

Spanish forces had searched a house and left, leaving Perez Garcia and two Iraqi policemen to complete the search, when a car arrived. The car sped off when its driver saw the house was occupied and Perez Garcia and the Iraqi policemen gave chase.

The occupants of the vehicle then opened fire on their pursuers, the Defence Ministry said.

Military commanders in Iraq say the number of attacks on their forces has dropped following the capture of Saddam last month. But security in Iraq remains precarious.

On Sunday, a suicide car bomb attack on one of the main entrance gates to the headquarters of the U.S.-led administration in Baghdad killed at least 25 people.

Facing a mounting death toll and spiralling financial costs of occupation, Washington wants to hand back sovereignty to an Iraqi government by the end of June, some four months before the U.S. presidential election.

But the US plan has been attacked by Iraq's most revered Shi'ite Muslim cleric because it does not allow for direct elections until 2005.

The United Nations is considering sending a team to Iraq to investigate whether earlier elections are possible, a move Washington hopes will soothe anger over its political roadmap.