Iraqi police say a suicide car bombing at a police station north of Baghdad has killed at least 13 people and wounded 15 others.

Officials say the bomber struck a group of recruits lining up outside the station in the town of Hibhib in restive Diyala province Thursday.

At the end of a Middle East tour, U.S. Defense Secretary Robert Gates acknowledged that U.S. officials underestimated the difficulty for Iraq's government to approve national reconciliation measures.

Gates told reporters on the flight home, after a visit to the United Arab Emirates, that the "depth of mistrust" among Iraq's Sunni Arab, Shi'ite and Kurdish leaders is much deeper than anticipated.

On Wednesday, Iraq's largest Sunni Arab political bloc announced it is withdrawing from Prime Minister Nouri al-Maliki's coalition government, further undermining attempts at national reconciliation.

A leader of the Iraqi Accordance Front, Rafaa al-Issawi, said Mr. Maliki's Shi'ite-majority government had failed to meet its demands, which included dealing with Shi'ite militias and reforming the Iraq security forces and detention procedures.

In other news, the U.S. military said coalition forces killed four insurgents and captured 20 other suspects during raids targeting al-Qaida in Iraq in Baghdad and Mosul on Wednesday and early Thursday. The military said the detainees include two alleged senior al-Qaida in Iraq leaders.

The U.S. military Thursday announced the deaths of three more American soldiers killed in combat in Iraq.

The military also announced the capture of what it called a "high value individual," who was detained in western al-Anbar province last week following a tip.

Some information for this report was provided by AP and Reuters.