There has been a spate of chlorine truck bomb attacks in recent months, mainly in western Anbar province. Ramadi is the capital of Anbar.

US commanders and Iraqi police have blamed al-Qaeda fighters for several of the chlorine attacks.

In Baghdad, a journalist from the US-funded Radio Free Europe/Radio Liberty was found dead in Baghdad with bullet wounds to her head and body, the broadcaster said on Friday.

RFE/RL, which has its headquarters in Prague, said in a statement that Khamail Khalaf, a reporter for the radio's Arabic service, was last seen on April 3. Her body was found in western Baghdad on Thursday.

In Diyala province, north of Baghdad, a roadside bomb blast killed one more US soldier and wounded two others, the military reported on Friday.

The soldier died on Thursday when the bomb exploded near his vehicle.

The latest fatality brought the military's losses in Iraq since the invasion to 3,262, according to an AFP count based on Pentagon figures.

Curfew

Also on Friday, Iraqi and US troops moved into the southern city of Diwaniya, a stronghold of Moqtada al-Sadr, the Shia leader, in an operation to curb the militia's increasing influence.

The US military said in a statement that Operation Black Eagle aimed "to disrupt militia activity and return security and stability of the volatile city back to the Iraqi government".

Residents and an Iraqi security source in Diwaniya told Reuters a curfew had been imposed and that troops were blocking streets and conducting house-to-house searches.

The security source said police in the city, many of whom are suspected of being infiltrated by Sadr's Mahdi Army militia, had been ordered to stay at home.

"There were clear instructions for local police not to report to work today. The security forces are from outside the city," the source told Reuters.

One person was killed and 19 wounded in the clashes that broke out between the Mahdi Army and soldiers overnight, a security official said.

At least 1,400 Iraqi soldiers arrived from neighbouring towns such as Kut, Babel and Najaf in order to raid gunmen hideouts in  Al-Askari, Al-Jumhuri, Al-Iskan in the northern parts of Diwaniyah,"  an Iraqi military officer said.

Diwaniya, 180km south of Baghdad, has witnessed fierce street battles between US-led forces and Mehdi Army militiamen in recent months.

The crackdown is focused on Baghdad, epicentre of the violence, but the government has said it will start spreading to other cities.

US and Iraqi forces have detained or killed hundreds of Sadr's supporters in the past few months.