An adviser to Iraq's prime minister said Thursday that radical Shiite cleric Muqtada al-Sadr is in Iran, but denied he fled due to fear of arrest during an escalating security crackdown.

Sami al-Askari said al-Sadr traveled to Iran by land "a few days ago," but gave no further details on how long he would stay in Iran. A member of al-Sadr's bloc in parliament, who spoke on condition of anonymity because of fear of reprisals, said al-Sadr left three weeks ago.

"I confirm that Muqtada al-Sadr is in Iran on a visit," said al-Askari. "But I deny that his visit is a flight."

The statement came a day after conflicting reports on al-Sadr's whereabouts. U.S. and Iraqi forces have increased pressure on backers of the anti-American cleric and other militants in a major security operation that began in force this week.

The chief U.S. military spokesman, Maj. Gen. William Caldwell, said al-Sadr "is not in the country" and that "all indications are, in fact, that he is in Iran." Al-Sadr's supporters have insisted he was still in Iraq.

It was not immediately clear whether al-Sadr's absence will lead to divisions among his loyalists, which include the Mahdi Army militia that has close ties to Iran. A splintering of his forces could lead to the emergence of smaller gangs vying for power among Iraq's Shiite majority.

Caldwell said U.S. authorities have been tracking al-Sadr's movements for months. He would not speculate on whether al-Sadr fled to escape the crackdown.

But the mercurial al-Sadr, who is not believed to have visited Baghdad in more than two years, often drops out of public view for weeks or months at a time. He failed to turn up for a planned speech Monday in the southern city of Najaf, where he lives, and has not been seen in public since Jan. 3.

He is believed to sleep in a different location every night to guard against attack.

When al-Sadr preaches at a mosque in Kufa, a town near Najaf, his security officers send out decoy convoys to confuse would-be attackers. His main fear is said to be an attack by rival Shiites, but he is also worried about the Americans and assassins hired by Sunni religious extremists who consider Shiites to be heretics.

Al-Sadr's militia is widely believed to receive Iranian money and weapons, but his relations with Tehran are not as close as are those of some Kurdish and Shiite parties allied with the Americans.

Aides to the cleric say he had secretly visited Iran at least once since 2003 in addition to a public visit last year.

Al-Sadr's family, one of the most prominent in the Middle East, won prestige by staying in Iraq during Saddam Hussein's dictatorship — unlike prominent Shiite figures from mainstream parties who sought refuge in Tehran during the 1980-1988 Iran-Iraq war.

Both al-Sadr's father and father-in-law were believed killed by Saddam's regime.

U.S. authorities have vowed to force al-Sadr's Mahdi Army militiamen off the streets and have pressured Prime Minister Nouri al-Maliki into telling al-Sadr that he could no longer protect his forces from the Americans, according to Iraqi officials.

Last week, U.S. and Iraqi troops raided the al-Sadr-controlled Health Ministry in Baghdad, arresting Deputy Health Minister Hakim al-Zamili. The U.S. accused al-Zamili of diverting millions of dollars to the Mahdi Army and allowing death squads to use ambulances and government hospitals for kidnappings and killings.

Al-Sadr rose from relative obscurity to become a national figure in the weeks after Saddam's ouster in 2003. His anti-American rhetoric and emphasis on his Arab ancestry — he claims descent from the Prophet Muhammad — have earned him the support of young and underprivileged Shiites across Iraq.

His top aides are mostly seminary students in their 30s and 40s who support him in part out of loyalty to his late father, Grand Ayatollah Mohammed Sadeq al-Sadr.

Al-Sadr ensures that none of his top aides become too influential or stay in the media limelight for too long. He has had several spokesmen and chief political aides abruptly pushed aside after they spent months in the public eye. Some of these, fearing the wrath of al-Sadr or his hardcore supporters, go into hiding.