The attacker detonated a bomb at the entrance to the base, which houses a jail, officials and security sources said. Taji is 20 km (12 miles) north of Baghdad.

The attack underscored the fragile state of Iraqi security as Washington pulls its remaining 14,500 troops out by year-end, nearly nine years after the invasion that ousted Saddam Hussein.

Violence has dropped sharply since the peak of sectarian slaughter in 2006-07. But Iraqi security forces still struggle to contain daily attacks by insurgents tied to Al-Qaeda and rival militias.

The Baghdad security operations center put the toll at 11 dead and 17 wounded. Police and hospital sources said 11 were killed and 19 wounded.

Militants launch scores of bombings and other attacks every month. According to official government figures, 161 civilians were killed in violence in October, the highest toll of the year, along with 97 police and soldiers.

Iraqi and US military officials have said Iraq may see an increase in attacks as American troops depart. Soldiers and police are frequent targets.

In July, two bomb blasts in the parking lot of a Taji municipal government building killed at least 28 people and wounded scores of others.

On Nov. 14 seven rockets landed in or near the US military’s Kalsu base near Iskandariya, 40 km (25 miles) south of Baghdad, wounding two Iraqi civilians living near the base, local police said.