TIKRIT, Iraq (Reuters) - Four suicide bombers stormed the police headquarters in a town north of Baghdad at dawn on Saturday, killing six Swat officers after a one-hour shootout, security sources said.

The bombers shot dead the guards outside the compound in the town of Baiji, 180km from Baghdad. One of them was killed in the ensuing fighting inside the base and the other three blew themselves up when army reinforcements arrived.

It was not clear who was behind the attack, but security forces and police are prime targets for Sunni Islamist militants who have been regaining momentum in their insurgency against Iraq's Shi'ite-led government.

Al-Qaeda's Iraqi and Syrian branches merged earlier this year to form the Islamic State of Iraq and the Levant, which has claimed responsibility for attacks on both sides of the border.

Intensifying violence has raised concerns of a return to wider conflict in a country where Shi'ite Muslims, Sunnis and ethnic Kurds have yet to find a stable way of sharing power.

About 800 Iraqis were killed in acts of violence in August, according to the United Nations.