A group of Islamic militants who disguised as police officers have killed at least 31 people including 14 police officers and wounded about 40 people in an overnight attack in the northern Iraqi city of Tikrit.

The militants wore police uniforms and used a police vehicle to enter the city, 175 km (110 miles) north of Baghdad, "there were around 10 attackers, including two suicide bombers",said in a statement.

The attacks targeted a police checkpoint and the house of a police colonel, who was killed with four members of his family

Two suicide attackers detonated their vests when surrounded by police, and three others were killed in separate clashes.

Five militants are thought to be hiding, Tikrit authorities had declared a curfew on Wednesday. Sporadic gunfire could be heard in the morning.

A U.S.-backed offensive is underway to dislodge Islamic State from the remaining districts under its control in Mosul, 225 km away, the group's last major urban stronghold in Iraq.

The militants overran about a third of Iraq's territory nearly three years ago, capturing most Sunni Arab cities located north and west of Baghdad, including Tikrit.

Iraqi forces and Iranian-backed Shi'ite militias drove the militants away from Tikrit two years ago. Tikrit is the home region of Saddam Hussein, the former president toppled in the 2003 U.S.-led invasion of Iraq.