A suicide bomber slammed a car packed with explosives into the gate of a prison north of Baghdad on Monday, killing at least 11 people, Iraqi officials said.

The attack in the town of Taji, about 20 kilometres north of the capital, is the third major attack in about a week in Iraq, and raises questions about the ability of the nation's security forces to protect the country after U.S. troops leave in just over a month.

A police officer said the attacker struck the main gate of al-Hout prison at 8 a.m. local time, when many employees and guards were on their way to work. Six policemen were among the dead and the rest were civilians, he added. At least 21 people were wounded.

Two health officials in nearby hospitals confirmed the casualty figures.

The officials spoke on condition of anonymity because they were not authorized to brief the media.

Violence has ebbed across Iraq since the height of the fighting, but deadly bombings and shootings still occur almost daily as U.S. troops prepare to leave.

Last Saturday a string of explosions hit a market in Baghdad and an area on the city's western outskirts, killing at least 15 people. Three days earlier, a triple bombing in the southern city of Basra killed 19 people.

Iraqi security officials maintain that they are fully prepared for the American withdrawal, which is required under a 2008 security pact between the U.S. and Iraq. About 15,000 U.S. troops remain in the country, down from a one-time high of about 170,000.

Many Iraqis, however, are concerned that insurgents may use the transition period to launch more attacks in a bid to regain their former prominence and destabilize the country.