Moqtada al-Sadr (2005 photo)

Aides to radical Iraqi cleric Moqtada al-Sadr are denying reports by U.S. officials that the Shi'ite leader fled to Iran two to three weeks ago.

Sadr aides said Wednesday that he is still in Iraq but had reduced public appearances for security reasons. Several aides said the cleric is in the Shi'ite holy city of Najaf, where his headquarters are located.

Tuesday, senior Bush administration officials told reporters that Sadr left by car for Tehran, as thousands of U.S. and Iraqi troops began moving into Baghdad to crack down on militias and insurgents. They said he is believed to still be in Iran.

The officials said that Sadr may have left Iraq, in part, because of a possible fracture between extremists in his Mahdi Army militia and his political bloc, which holds 30 seats in Iraq's parliament.

Meanwhile, the U.S. military Wednesday confirmed that a Marine helicopter that crashed in Al Anbar province last week was brought down by hostile fire.

Earlier, the military suggested mechanical failure might have caused the crash that killed all seven American service personnel on board.

Bush administration officials have accused Iran of smuggling weapons into Iraq for Shi'ite militias. The administration has repeatedly warned Iran against fueling violence in Iraq, and U.S. forces have detained a number of Iranian officials in recent raids.

Also on Tuesday, the Iraqi commander of the Baghdad security crackdown said his country will seal its borders with Iran and Syria for three days. Iraqi Lieutenant General Abboud Gambar did not say when the border closures will begin.