"We have seen the reports and believe them to be accurate," the official told AFP in Baghdad, referring to reports in the US media that the firebrand preacher had driven to Tehran ahead of a ramped-up security plan.

The Pentagon describes Sadr's Mahdi Army militia as the most dangerous single faction in the vicious sectarian war gripping Iraq, accusing rogue elements of the force of killing hundreds of Sunni civilians.

The defence official spoke on condition of anonymity, as has become typical when US military sources make allegations against local and Iranian leaders.

Media reports quoted US intelligence officials as saying Sadr had left Iraq two to three weeks ago to avoid being killed or captured by US troops carrying out a new operation to put and end to sectarian violence.

But Nassar al-Rubaie, the head of Sadr's parliamentary bloc, insisted he was "still inside Iraq and working normally" but would not say exactly where.

And Bassem al-Aathari, an official at Sadr's office in Najaf south of Baghdad, said the cleric is still in the Shiite holy city and that if he was to travel abroad this would be announced as had been the case for previous trips.

"It's ridiculous. It's nothing," Rubaie told AFP in Baghdad.

"If the US army already in Iraq is incapable of resisting Sadr, what difference would 20,000 reinforcements make?" he demanded.