Unknown assailants fired mortars on an Iraqi National Guard base in Mashahda, a town about 40 km north of Baghdad. The interior ministry said four people were killed and 82 wounded.

The US-led military confirmed an attack on the facility but said no US soldiers had been hurt.

Army helicopters were used to shuttle the wounded to a military hospital, it said in a statement.

Iraq's fledgling security forces have been a prime target for insurgents in the country in the aftermath of last year's US-led invasion.

The US-led military is another favourite target and it also suffered a direct hit on a compound in Baghdad that left one US contractor dead and seven people injured, including a US soldier, a spokesman said.

"We had mortar and RPG (rocket propelled grenade) fire that hit inside our compound this morning," a military spokesman told AFP.

The US soldier and one of the six Iraqi civilians injured in the attack were in a serious but stable condition, he said.

The Iraqis nationals had all been working in the compound.

The attacks came less than a week after bombings killed five people in the Green Zone, the previously impregnable compound in Baghdad housing Iraqi government offices and the US embassy.

Kellogg Brown and Root (KBR), a subsidiary of the US oil services giant Halliburton, confirmed its employee was killed, bringing to 54 the number of deaths suffered by Halliburton and its subcontractors in Iraq.

"KBR regrets to confirm the death of one employee who was killed today as a result of injuries sustained during a mortar attack near Baghdad," it said in a statement.

In Samarra two Iraqis were killed in fighting between US troops and insurgents as the Americans carried out sweeps around the northern town of Duluiya, considered a hotbed of Islamic extremism, medical sources said.

A US military spokesman said soldiers had been sealing off neighborhoods and conducting searches for several days and had carried out detentions in the town, 75 kilometers (40 miles) north of Baghdad.

"Two Iraqi corpses and six wounded were brought here," said Doctor Mohamed Jaafar Moahmed at the general hospital in neighboring Balad.

At least 12 people had been arrested by the Americans in the sweeps, said local council member Talal Kahtan.

Meanwhile, an oil pipeline in an area north of the capital was ablaze on Tuesday, the US military said, suggesting it may have been attacked by saboteurs.

Soldiers discovered a fire on a pipeline north of a refinery in the town of Baiji early Tuesday, US military spokesman Robert Powell said, adding: "The oil pipeline was shut down until the fire is put out and repairs have been made."

Asked about the possible cause of the blast, he said: "I'm sure they are looking in the possibility of an IED (improvised explosive device)."

Earlier the British-born head of CARE International's Iraq operations, Margaret Hassan, was kidnapped in Iraq early Tuesday, the aid organization said in London.

Hassan, who is married to an Iraqi and is a naturalized Iraqi citizen, has worked for CARE in Iraq since 1992 and has lived for more than 30 years in the country, a CARE spokeswoman told AFP.