"The policemen were travelling in a car when they were stopped on the highway between Kirkuk and Tikrit by unidentified gunmen. The four were shot dead by the gunmen," said Captain Faras Mahmoud, a police officer in Kirkuk.

Abdullah Khir Allah told AFP that his 21-year-old son Mohammed had just qualified as a police officer and was travelling south from the Kurdish town of Erbil to the city of Samarra to see his future bride.

Three of his colleagues were travelling with him as escorts, but when they neared Tikrit -- ousted Iraqi leader Saddam Hussein's hometown -- the Kurdish Shiite officers were ambushed by suspected Sunni Arab insurgents.

"We worked so hard to bring him up. His mother was so busy with him until he went to the academy, and he was to be married," Khir Allah said bitterly, cursing the attackers and accusing US troops of failing to provide security.

In another attack in southern Kirkuk, a suicide car bomber blew himself up close to the office of President Jalal Talabani's party, the Patriotic Union of Kurdistan, killing one guard and wounding 16 party members, police said.

The bomber rammed the fence of the office and blew up the car. The explosion also damaged 12 other vehicles.

Last week the party's office in Mosul was similarly targeted, killing eight Peshmerga militiamen guarding the building and wounding 51.

Kirkuk, an ethnically mixed town and a centre of the Iraqi oil industry, has seen a number of insurgent attacks in the past few months. Sunday's shooting took place 50 kilometres (31 miles) south of the city.