Iraqi and US forces clashed with Shi'ite militia loyal to radical cleric Moqtada al-Sadr as they launched a major operation at dawn on Friday to return the volatile city of Diwaniya to government control.
In Ramadi, west of Baghdad, a truck bomb killed at least 15 people and spewed chlorine gas into the air, Iraqi police Colonel Tareq al-Dulaimi said, in the latest in a string of attacks involving the poisonous chemical. Another police captain said five people were killed.
An Iraqi security source said Iraqi and US troops fought militiamen in districts in southeast Diwaniya, a stronghold of Sadr's Mehdi Army, accused by Washington of driving Iraq towards civil war through attacks on Sunni Muslims. A local hospital said at least one person had been killed and 15 wounded.
"Iraqi army soldiers swept into the city of Diwaniya early this morning to disrupt militia activity and return security and stability of the volatile city back to the government of Iraq," the US military said in a statement.
The Iraqi government said this week it was extending Operation Imposing Law, a major security crackdown in Baghdad aimed at curbing rampant sectarian bloodshed that threatens civil war, to areas outside the capital.
The United States regards Sadr, a key political ally of Prime Minister Nuri al-Maliki, and his Mehdi Army militia as the greatest threat to peace in Iraq.
The Mehdi Army is a powerful force in Diwaniya, 180 km (110 miles) south of Baghdad, where US and Iraqi forces have fought militiamen in past months. Forty people were killed in fierce fighting in street battles in October.
Thirteen Iraqi soldiers were summarily executed when they ran out of ammunition and were captured during a firefight with Shi'ite militiamen in the town last August. The incident prompted questions about the capabilities of the new Iraqi army.
While the operation has succeeded in reducing the death rate in Baghdad, there has been an explosion of violence elsewhere. The Iraqi government says militants forced out of the city have turned other areas into new "killing fields".