BAGHDAD (AFP) - Gunmen attacked a convoy carrying the head of Iraq's anti-corruption watchdog north of Baghdad on Saturday, sparking clashes that killed nine policemen, an officer and a doctor said.

The attack on the convoy carrying Alaa Jawad, who was unharmed in the attack, also wounded 21 people and took place on the road between Baghdad and the city of Samarra.

It came as security forces fight to retake areas north of the capital seized by militants in a major offensive they launched on Monday night.

It was not immediately clear who was behind the attack, or if it was planned or a target of opportunity.

The militant offensive, spearheaded by jihadists from the Islamic State of Iraq and the Levant but also involving supporters of late dictator Saddam Hussein, overran all of one province and chunks of three others.

But while security forces initially performed poorly, they appear to have overcome some of the initial shock of the onslaught, and have retaken several areas north of the capital.