WASHINGTON — The radical Shiite cleric Moqtada al-Sadr fled Iraq for Iran ahead of a security crackdown in Baghdad and the arrival of 21,500 American troops sent by President Bush to quell sectarian violence, a senior American official said yesterday.

Mr. Sadr left his Baghdad stronghold some weeks ago, the official said, and is believed to be in Tehran, where he has family. The official, speaking on condition of anonymity to discuss American monitoring activities, said fractures in Mr. Sadr’s political and militia operations may be part of the reason for his departure. The move is not believed to be permanent, the official said.

Word of Mr. Sadr’s departure coincides with an announcement that Iraq will close its borders with Iran and Syria for 72 hours as part of the drive to end the violence that has threatened to divide the capital along sectarian lines.

The American official said it is not clear how firmly the radical Shiite cleric is controlling his organization and the associated Mahdi army militia from exile.

“The question for us is to what extent his organization is going to participate in the political process,” the official said, referring to Mr. Sadr’s on-again, offagain relationship with the fragile democratic government in Baghdad.

Mr. Sadr’s departure was reported by several television networks yesterday.

Mr. Sadr’s militia is widely seen as the main threat to Iraq’s unity and high on the list of targets for the Baghdad security operation.

A ragtag but highly motivated militia that fought American forces twice in 2004, the Mahdi army is blamed for much of the sectarian strife shaking Iraq since a Shiite shrine was bombed by Sunni militants a year ago. American officials have for months pressed Shiite Prime Minister al-Maliki to move against the militia, but he has so far done little to comply, largely because he does not want to lose Mr. Sadr’s support.

Mr. Sadr rose from obscurity in the aftermath of the ouster of Saddam Hussein to lead a movement of young, underprivileged Iraqis united by opposition to America’s military presence as well as hunger for Shiite domination.

The cleric, who is in his mid-30s, is a master of street politics, and his young lieutenants can rally tens of thousands of protesters at short notice. Once wanted in the 2003 killing of a key cleric, Mr. Sadr gained much influence when his parliamentary bloc of 30 of 275 deputies was instrumental in Mr. Maliki’s election.

Dismissed by older Shiite politicians as a dangerous upstart, Mr. Sadr set up the Mahdi army militia in 2003. It is suspected of being behind the abduction and murder of thousands of Sunnis in what are known as death squad killings.

Two key members of Mr. Sadr’s political and military organization were gunned down last week, the latest of as many as seven key figures in the Sadr organization killed or captured in the past two months.

The deaths and captures came after Mr. Maliki, also a Shiite, dropped his protection for the organization.

Shiite leaders insist that the Shiite militias flourished because America and its allies could not protect civilians. They say if the Sunni insurgents were crushed, the threat from Shiite hard-liners would go away.

Shiite politicians have long maintained that Sunni militants pose a greater threat to Iraq’s stability. Thousands of Shiite civilians have been killed in bombings and suicide attacks carried out by Al Qaeda in Iraq and other Sunni groups.

Thousands regularly cross the porous Iraq-Iran border, and Iran has been a popular destination for elite Shiite Iraq exiles. In Saddam’s time those exiles included Mr. Maliki, who like other educated and politically active Shiites feared for his safety in Iraq.