The explosion of a roadside bomb led to the death of five American soldiers Monday in Iraq’s northern Ninevah province, described as one of al-Qaeda in Iraq’s last strongholds. The event occurred days after a massive explosion and suicide attack killed 40 people in the provincial capital.

The attack was in Mosul, Iraq’s third-largest city, 390 km north of Baghdad. The attack against the US patrol raises the number of American soldiers killed in Iraq to 36 this month, up from 23 in December but similar to levels in October and November. The number of US troops killed since 2003 invasion now stands at 3,940.

On Thursday, Brigadier General Saleh Mohammed Hassan, the director of police for Ninevah province, and two of his officers were assassinated as they toured the wreckage by a man wearing an explosives vest.

On Friday, Iraqi Prime Minister Nouri Maliki announced the start of a major offensive in Mosul against al-Qaeda to push the Sunni Islamist group out of the city, its last major urban stronghold. Therefore, on Sunday, Iraqi army reinforcements moved into positions near Mosul.

The U.S. military said the cause of the blast has yet to be determined, although Iraqi officials were quick to blame al-Qaeda.