The volunteers, armed with Kalashnikov assault rifles and pistols, have been guarding Sadr's home since the assassination of prominent Shiite leader Ayatollah Mohammad Baqer al-Hakim in a car bomb massacre in this central pilgrimage city last Friday.

"Today around 3:00 pm (1100 GMT), the Americans came up to Sadr's house in three armoured cars and demanded that the special guard hand over its weapons but they refused," spokesman Qais al-Khazali told AFP.

"The Americans left and came back with Iraqi police officers who in turn demanded that the guards disarm but they too were rebuffed," he said.

"The Americans came again but, seeing a large number of Sadr supporters around the house, they left, perhaps to get reinforcements."

The US army did not immediately comment on the incident, but a Marine in Najaf, who declined to give his name, confirmed that a patrol had gone by Sadr's house.

But he insisted the US soldiers did not ask for any weapons.

"We've worked the last four and a half months to bring stability to this place and the last three to four days have shot it all to hell," he commented.

About 200 men, many of them armed, gathered outside Sadr's house late Wednesday afternoon.

A group of armed men at one point rushed at a four-wheel-drive vehicle that approached the building, warning it to leave the area, which it did.

Friday's deadly car bombing, which also killed 82 other people, has sparked calls from several Shiite leaders for militias to take charge of security but they have been firmly rejected by the Americans.

"We believe that there is no role in the new Iraq for organized militias," US civil administrator Paul Bremer told a news conference Tuesday, adding that the US-led coalition was nonetheless willing to provide weapons and uniforms for a special police force to protect the Shiite holy places.

On Sunday, Sadr's guards killed two people and seriously injured two more when they opened fire on a car passing the front of his house late at night.

WAR.WIRE