Fourteen of the injured were taken to the U.S. Army's 31st Combat Support Hospital and seven were treated at a clinic on the U.S. base, known as Camp Anaconda, according to a military statement.

Air and ground units responded to the attack, the military said. Balad is 50 miles north of Baghdad.

The military statement did not specify whether the injured were U.S. soldiers or included civilians or others on the sprawling compound.

On June 6, a U.S. soldier on the same base was killed and another wounded in a mortar attack. Camp Anaconda was also the scene of a mortar attack last July 4 that wounded 18 U.S. soldiers.

Also yesterday, saboteurs blasted a key southern pipeline for the second time in as many days, shutting down Iraq's oil exports, and gunmen killed a security chief for the state-run Northern Oil Co.

The latest attacks at Iraq's oil sector have slowed the process of reviving its economy after decades of war, international sanctions and Saddam Hussein's tyranny. Insurgents also are targeting the country's infrastructure apparently to undermine confidence in the new government, which takes power from the U.S.-led coalition June 30.

Elsewhere, radical Shiite cleric Muqtada al-Sadr ordered members of his militia to leave the holy cities of Najaf and Kufa unless they live there, fulfilling a key aspect of an agreement meant to end fighting between his forces and U.S. troops.

Exports were halted last month through the other export avenue the northern pipeline from Kirkuk to Ceyhan, Turkey after a May 25 bombing, Turkish officials said on condition of anonymity.

Two explosions on the southern pipeline occurred in the same area as a blast Tuesday. It could take up to a week to repair, Jassim said.