BAGHDAD - A car bomb explosion killed at least 15 people and wounded 25 on Thursday in Iraq's predominantly Shi'ite southern city of Hilla, medical and police sources said.

A suicide bomber rammed his car into the entrance of a police headquarters in Hilla, 100 km (60 miles) south of Baghdad, during a shift change when many police officers were outside the building, the sources said.

An Interior Ministry source in Baghdad put the toll at 16 killed and 50 wounded.

Iraqi security forces went on high alert for revenge attacks after US commandos killed al Qaeda leader Osama bin Laden.

US and Iraqi officials say al Qaeda in Iraq has been severely degraded in recent years.

But eight years after Saddam's overthrow, Iraq still faces a lethal insurgency that carries out dozens of bombings and other attacks each month, many of them on Iraqi police, soldiers and government officials.

Hilla was hit by one of the deadliest attacks in Iraq in May last year when two suicide car bombers drove into the entrance of a textile factory as workers were ending a shift, killing at least 35 people and wounding 135. A third bomb exploded as police and medics rushed to the scene.

On Monday four people were wounded when a sticky bomb attached to a car in a parking lot exploded in Hilla.