Baghdad: Tension prevailed in Iraq on Thursday as supporters of radical Shiite cleric Moqtada Al Sadr attacked offices of a rival Shiite group in several cities after deadly clashes in the holy city of Najaf.

CNN reported that US troops have headed for Najaf to stabilise the situation.

The fresh outbreak of inter-Shiite violence has cast clouds on Iraq's constitutional process.

A final draft of the charter is due to be put before parliament for approval on Thursday.

One man was killed and 13 wounded in overnight clashes between Sadr supporters and members of the rival Supreme Council for the Islamic Revolution in Iraq (SCIRI) in the southern town of Nasiriyah, 375 kilometres (230 miles) southeast of Baghdad.

In Baghdad, the Mahdi Army, the militant wing of Sadr's movement, had taken control of Shiite stronghold of Sadr City. Though unarmed, the militia was seen moving on the streets with no sign of police and army.

Trouble in the south began when supporters of Al Sadr tried to reopen his office in Najaf, which was closed after fighting there last year.

When Shiites opposed to Al Sadr tried to block the move, fights broke out. Four people were killed, 20 were injured and Al Sadr's office was set on fire, police said

The confrontation on Wednesday in at least five southern cities followed the boldest assault by Sunni insurgents in weeks in the capital.

Dozens of insurgents wearing black uniforms and masks attacked Iraqi police in western Baghdad with multiple car bombs and small-arms fire that killed at least 13 people and wounded 43, police said.

The new violence came as US Defense Secretary Donald Rumsfeld ordered two battalions from the Army's 82nd Airborne Division to deploy to Iraq for 120 days to beef up security for the elections.

The New York Times reports that the Najaf fighting spurred members of Al Sadr's militia, the Mahdi Army, to storm the Baghdad offices of a competing Shiite party led by Prime Minister Ebrahim Al Jaafari, officials said.

Al Jaafari heads the country's biggest Shiite party, the Supreme Council for the Islamic Revolution in Iraq, or SCIRI. The party, which controls key posts in the national government, quickly denied responsibility and condemned the attack.

As word of the Najaf attack spread, clashes broke out between the rival groups across central and southern Iraq. The violence extended to the country's second largest city, Basra, where several hardline Shiite groups are competing for influence.

Interior Minister Bayan Jabr, a member of SCIRI, told Iraqiya television late Wednesday that he was dispatching a commando brigade to Najaf to restore order. A curfew was imposed from 11 p.m.

Despite the government's move, 21 pro-Al Sadr members of parliament and three senior Cabinet officials announced they would refuse to perform their duties indefinitely to protest the Najaf attack. Municipal officials loyal to Al Sadr in several southern cities issued similar declarations.