U.S. and British backing was limited to air support and teams of mentors embedded with Iraqi officers, Bergner said.
WITHDRAWAL PLANS
Washington aims to bring 20,000 of its 160,000 troops home by July after a build-up of troops improved security last year. U.S. Democratic candidates who aim to succeed President George W. Bush next January are calling for a faster withdrawal.
But violence has increased in the past few months and Iraqi forces have yet to show they can tackle militants on their own.
Sadr, a young, anti-American cleric, helped install Maliki in power after an election in 2005 but later broke with him. His followers, known as the Mehdi Army, have feuded with other Shi'ite groups seen as influential in Maliki's government.
Sadr declared a ceasefire last August, winning praise at the time from U.S. commanders for helping to reduce violence, although they say "rogue" Mehdi Army units outside Sadr's control have fought on with support from Iran.
Despite the violence, Sadr aides said the cleric's truce was still formally in place, a negotiating posture that could be useful for Sadr in coming days.
Sadr's followers have taken to the streets demonstrating against Maliki's government and forcing schools, universities and shops to close. On Tuesday he said he would call a "civil revolt" if attacks on his followers did not stop.
The head of Sadr's office in Basra, Harith al-Ithari, said the movement was negotiating with Maliki to end the fighting.
"There are ongoing negotiations with the prime minister. Maliki asked to meet Sadr officials in Basra," he told Reuters.
Another top aide, Hassan al-Zargani, read to Reuters what he said was a statement from Sadr calling on Maliki to leave Basra and appoint a delegation to hold talks.
Sadr has long been a thorn for rival Shi'ite groups in Maliki's circle, said Iraqi political analyst Hazem al-Nuaeimi.
"There is a need to minimise the Sadrists' strength and influence and to draw the lines before they get any stronger."
British forces, which patrolled Basra for nearly five years, withdrew to a base outside the city in December and were not involved in the fighting. A British military spokesman said the Iraqi assault was expected to last two to three more days.
An official with Iraq's Southern Oil Company said production in the Basra area which produces 80 percent of Iraq's exports could be disrupted if fighting lasted more than three days.
