The attack follows warnings from Shiite militants backed by Iran and anti-American cleric Muqtada al-Sadr that they would violently resist any effort to keep American troops in Iraq past their year-end deadline to go home.

Although American casualties have dropped considerably in the two years since U.S. troops pulled back from Iraq cities, Shiite militias have begun hammering U.S. bases and vehicles with rockets, rocket-propelled grenades and roadside bombs over the past three months.

The militants' goal appears twofold: to give the impression that they are driving the withdrawing U.S. forces out of Iraq, and to make the U.S. think long and hard before agreeing to any Iraqi request to keep a contingent of troops in this country beyond the end of the year.

"Iranian-backed militias are flexing their muscles and have steadily increased military pressure on U.S. forces since rumors first started in the early spring concerning an extension of the U.S. presence," said Michael Knights, an Iraq analyst at the Washington Institute.

AP