A homicide car bomber rammed an explosives-laden vehicle against the wall of a police station south of Mosul on Thursday, killing three policemen and wounding four others, authorities said.

It was the fifth homicide attack in Iraq this week and showed that insurgents can still carry out assaults despite security gains in urban areas of the country. Four homicide bombers killed 57 people in Baghdad and the northern city of Kirkuk on Monday.

The Thursday attack occurred on a police station in the Qayara area about 30 miles south of Mosul, according to a police officer who spoke on condition of anonymity because he was not authorized to speak to the media.

A statement posted Wednesday on a Web site in the name of the Islamic State of Iraq, an Al Qaeda front group, warned of a campaign of attacks in the Mosul area in retaliation for the killing of one of its "hero brothers."

U.S. and Iraqi forces have been battling Al Qaeda-inspired insurgents for months in Mosul, where militants have carried out numerous car bombings and assassinations.

On Thursday, a judge died of wounds suffered in an attack the day before in Mosul, police said. One of the judge's bodyguards was killed in the attack, police said.

The focus of the war against Al Qaeda and other Sunni militant groups has shifted to the north after the Sunni revolt against the terror movement in Anbar province west of Baghdad. Sunni militants have also suffered major losses in the Baghdad area since 2006.

About 50,000 U.S.-backed Iraqi military and police forces have launched a major operation against Al Qaeda insurgents in Diyala province northwest of the capital. Iraqi officials say Diyala is one of the last major Al Qaeda strongholds near the capital.

Associated Press Television News footage on Thursday showed Iraqi troops transferring a handful of detainees in the back of a truck in Diyala. One of the soldiers was seen feeding a blindfolded captive with a slice of orange as others were giving water to them.

Also Thursday, the U.S. military said American soldiers wounded a civilian woman after opening fire on a group of four suspected militants during an operation in Samarra, north of Baghdad.

The military said another woman was also killed in the confrontation but her status had not been determined.