The six were returning to the oil city of Kirkuk, 250km north of Baghdad, from a meeting at the Baiji refinery, Iraq's biggest, when they were stopped about 30km from Kirkuk, on Friday.

No further details were available.

Security problems in the Kirkuk area, including frequent attacks on pipelines, have hindered Iraq's ability to exploit the vast potential of the northern oilfields.

Nearly all of Iraq's oil exports - averaging below two million barrels a day - have come from the southern fields around Basra.

US soldiers killed

Also on Friday, a roadside bomb killed three American soldiers south of Baghdad as US and Iraqi forces swept through a city to the north where three fighters were killed the day before after firing on American troops.

The three Americans died in the attack shortly before noon in Babil province, the US military said, giving few other details.

The latest attack raised the US
casualties in Iraq to at least 2414

However, Iraqi police said the blast occurred against a military convoy near Mahaweel, 55km south of Baghdad.

The attack raised to at least 2414 the number of US military members who have died since the Iraq war started in March 2003, according to an Associated Press count.

Bodies found

In Samarra, 95km north of Baghdad, American and Iraqi forces imposed a daytime curfew and searched selected neighbourhoods looking for insurgents a day after three fighters were killed after they opened fire on US soldiers, police said.

Also on Friday, the bodies of eight Iraqis who apparently were kidnapped and killed also were found, four in Baghdad and one on the outskirts of the city, police said.

An Iraqi police major was assassinated in a drive-by shooting in Diwaniyah, 130km south of Baghdad, police said.

A Shia cleric, Hussein Ahmed al-Mousawi, was shot dead near his home in southwestern Baghdad's Dora district where tensions between Sunnis and Shia run high.