On Monday a suicide car bomber struck a U.S. patrol in northern Iraq killing four American soldiers and an Iraqi interpreter.

This is the deadliest single attack against U.S. forces in nine months.

The blast occurred as U.S. vehicles were passing near an Iraqi police checkpoint in Mosul, Iraq's third largest city and the last major urban battleground in the war against al-Qaida and other Sunni insurgents.

American casualties have fallen to some of their lowest levels of the war since thousands of Sunnis abandoned the insurgency and U.S. and Iraqi forces routed Shiite militias in Baghdad and Basra last spring. Only five of the 16 U.S. service members who died in Iraq last month were killed in action.

Fighting however continues in Mosul and elsewhere in northern Iraq — a conflict which U.S. officials say is driven in part by ethnic rivalries between Sunni Arabs and Kurds. Many Sunni extremists are believed to have fled north after being driven from longtime strongholds in Baghdad and central Iraq.

A U.S. statement said three U.S. soldiers were killed at the scene of Monday's attack. A fourth soldier and the interpreter died of wounds at a military hospital, the U.S. said.

An Iraqi police official, who spoke on condition of anonymity because he wasn't authorized to release the information, said two Iraqi policemen and one civilian were wounded.

It was the deadliest single attack against U.S. troops since May 2, 2008, when four Marines were killed in a roadside bombing in Anbar province, a former insurgent stronghold west of Baghdad.

Iraq U.S.