The attack was the seventh suicide bombing in Iraq in one month, indicating Sunni insurgents are seeking to expand their campaign to stoke tensions against the Shi'ite-led government of Prime Minister Nuri al-Maliki.

Sunni tribal fighters from the Sahwa militia - Sons of Iraq - were gathering at an office in Taji, 20 km (12 miles) north of the capital, when the bomber detonated his explosives among them, police said.

Sahwa are former insurgents who turned against al-Qaeda's local wing in the Sunni heartland province of Anbar at the height of the US-led war and helped American troops swing the tide of the conflict in the area.

Al Qaeda's affiliate, Islamic State of Iraq, last year vowed to take back ground lost during its long war with American and US forces, and has urged Iraqi Sunnis to rise up against Maliki's government.

The spike in suicide attacks comes as Maliki faces mass protests by Sunni Muslims demanding an end to what they see as marginalisation of their minority sect since the fall of Saddam Hussein and the rise of Iraq's Shi'ite majority.