Anti-American cleric Muqtada al-Sadr fled Iraq for Iran ahead of a security crackdown in Baghdad and the arrival of 21,500 US troops sent by President George W. Bush to quell sectarian violence, a senior US official said. Al-Sadr left his Baghdad stronghold some weeks ago, the official said Tuesday, and is believed to be in Teheran, Iran's capital, where he has family. The official, speaking on condition of anonymity to discuss US monitoring activities, said fractures in al-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 al-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 Baghdad along sectarian lines.