Baghdad - Three U.S. soldiers and an interpreter were killed Monday during fighting north of Baghdad, the military announced.

    The combat took place in Diyala province, an area northeast of Baghdad that remains volatile despite an overall drop in violence nationwide.

    The statement did not provide more details.

    The attack came two weeks after a suicide car bomber struck a U.S. patrol in the northern city of Mosul, killing four American soldiers and an Iraqi interpreter in the deadliest single attack against U.S. forces in nine months.

    At least 4,250 members of the U.S. military have died in the Iraq war since it began in March 2003, according to an Associated Press count.

    Iraq's Interior Ministry, meanwhile, announced the arrest of a Shiite police gang accused of killing the Sunni vice president's sister in 2006 as part of a string of kidnappings and slayings.

    Spokesman Maj. Gen. Abdul-Karim Khalaf said the 12 people arrested were former employees of the ministry. The Interior Ministry has been accused of past infiltration by Shiite militias who carried out some of the worst sectarian violence.

    The sister of Vice President Tariq al-Hashemi, Maysoun al-Hashemi, died in a hail of gunfire on April 27, 2006 as she left her home in Baghdad.

    Abdul-Karim al-Samarraie, a member of al-Hashemi's Iraqi Islamic Party, welcomed the arrests.

    "It is a good thing that the Interior Ministry succeeded in arresting Maysoun al-Hashemi's killers ... although it took too long to find them," he said.

    Also Monday, gunmen ambushed an Iraqi army checkpoint in western Baghdad, killing three soldiers and wounding eight other people, according to police.

    And in central Baghdad, a roadside bombing apparently targeting a police patrol killed at least two civilians and wounded six, said police and hospital officials.

    The officials spoke on condition of anonymity because they weren't authorized to release the information.