US forces backed by dozens of tanks and Bradley fighting vehicles have overrun Baghdad's suburb of Sadr City, a powerbase for Moqtada Al Sadr, witnesses say.

Soldiers in Humvees called on militiamen, who appeared to have stopped fighting and melted away, to surrender their weapons. The thrust, which began yesterday, marks the biggest push against Sadr's Mehdi Army militia in Baghdad.

"Sadr City residents, your government has banned all militias in Iraq. Come forward and surrender your weapons," a message in Arabic read by loudspeakers from Humvees said.

Armoured vehicles were deployed in all main streets in the suburb, a slum of two million people that has long been a no-go area for the US military.

Soldiers cleared suspected explosive charges and unexploded munitions as some residents stood on sidewalks watching them.

The US forces had advanced into the suburb on Wednesday, meeting sporadic resistance. The military said it killed 50 militiamen in that advance.-Reuters