Meanwhile, Zarqawi's group, responsible for many car bombings and beheadings of hostages, claimed responsibility for a failed bid on Monday to assassinate a leading Shi'ite politician, according to an Islamist website.

Also on Monday, rebels stormed a police station in Dijla, between Tikrit and Samarra, and executed 12 police officers before dynamiting the building, police said.

Militants released a videotape yesterday, saying they had executed eight and released two Iraqis employed by Sandi Group, an American security company. They had been held hostage since December 13. The claim could not be independently verified.

The insurgents claiming to represent three Iraqi militant groups - the Mujaheddin Army, the Black Banner Brigade and the Mutassim Bellah Brigade - said in the tape that "the eight have been executed because it was proven that they were supporting the occupational army".

Just outside Tikrit, the home town of deposed president Saddam Hussein, three police officers were killed in an attack on a checkpoint, police said.

Another four police and a national guardsman were shot dead at a police station in Ishaki, south of the formerly rebel-held city of Samarra, they said.

Three guardsmen and three civilians were killed in a car-bomb attack targeting a joint US-Iraqi convoy in Samarra, hospital sources said.

In Baquba, 50km northeast of the capital, six national guardsmen were killed in a suicide bombing, medics said.

South of the main northern city of Mosul, two police were killed in the town of Shorgat, an officer said.

In the same area, an Iraqi interpreter for the US army was killed and an Iraqi businessman travelling with him was kidnapped, another police officer said.

Further south, near the refinery town of Baiji, a hospital official said a roadside bomb killed one Iraqi civilian and wounded another.

And at Suleyman Beg, northeast of Samarra, three Iraqi businessmen working with the US occupation forces were killed and a curfew was imposed on the town, an officer said.

Another policeman was killed in Balad, north of the capital, when insurgents opened fire on security forces guarding a voter-registration centre, police said.

And west of Baghdad, in the rebel stronghold of Ramadi, the body of deputy provincial governor Muyad Marwan al-Ithawi was found riddled with bullets hours after his kidnapping by gunmen.