Iraqi authorities say the death toll from five days of intense fighting between Iraqi forces and Shi'ite militias has surged to more than 200.
Many of the casualties have occurred in Baghdad's Sadr City, a stronghold of militias loyal to radical Shi'ite cleric Moqtada al-Sadr. A spokesman for the Iraqi Health Ministry says at least 75 people have been killed there and nearly 500 wounded in clashes and U.S.-led airstrikes.
Reports say dozens of policemen in Sadr City surrendered their weapons to Sadr's forces, saying they could not fight their own people.
To the south, a British military spokesman, Major Tom Holloway, says U.S. warplanes bombed militia strongholds in a neighborhood just north of the city of Basra. There was no immediate word on casualties.
Witnesses say eight people were killed in a U.S. airstrike earlier Saturday in Basra.
Fighting between Iraqi forces and Sh'ite militias has intensified since Tuesday, when Prime Minister Nouri al-Maliki ordered a crackdown on militants in the southern city of Basra.
On Friday, U.S.-led coalition aircraft joined Iraqi military ground forces for the first time in their effort to crack down on the militias.
Prime Minister Maliki has extended until April 8 a deadline for militant fighters to surrender their weapons, in exchange for money. The militias have told foreign news agencies they will not accept Mr. Maliki's offer.
President Bush said Friday the fighting against the Shi'ite militias represents a "defining moment" in the history of a free Iraq. He calls the developments a necessary part of the development of a free society.
Militants have been bombarding the Green Zone since Sunday. At least two Americans have been killed in the attacks, and the U.S. State Department has urged all U.S. Embassy personnel to stay inside reinforced buildings.
Some information for this report was provided by AFP, AP and Reuters.