Defense Secretary Robert Gates said Friday he doubts that radical Shiite cleric Muqtada al-Sadr, whose Mahdi Army is battling U.S. and Iraqi troops in Baghdad, would be subject to arrest by U.S. forces.

Al-Sadr is believed to be in Iran while elements of his militia in the Sadr City section of the capital fight Iraqi government troops supported by the U.S. military.

Gates was asked at a Pentagon news conference about the prospect of al-Sadr being arrested. He initially responded in broad terms about the U.S. view of competing political players in Iraq.

"I think those who are prepared to work within the political process in Iraq, and peacefully, are not enemies of the United States," Gates said.

When pressed about the prospect of arresting al-Sadr, he added, "I would be surprised along those lines — a move to arrest him. He is a significant political figure. We want him to work within the political process. He has a large following. It is important that he become a part of the process, if he is not already."

A senior aide to al-Sadr was assassinated Friday in the holy city of Najaf, officials said. Authorities immediately announced a citywide curfew and security forces were seen deploying on the streets. The killing threatened to raise tensions amid a violent standoff between al-Sadr's Mahdi Army militia and the U.S.-backed Iraqi government.

Adm. Michael Mullen, the chairman of the Joint Chiefs of Staff, appearing alongside Gates at the news conference, said he regards al-Sadr as "somewhat of an enigma."

"So, I think Sadr clearly is a very important and key player in all this," Mullen added. "Exactly where he's headed and what impact he'll have long term, it's, I think, is out there still to be determined."

Gates reiterated that he hopes it will be possible to withdraw more U.S. forces this fall after a pause this summer.

"I certainly hope, continue to hope, that conditions will allow us to remove more troops by year's end," he said. "That hope for return on success is shared by the president, General Petraeus, Admiral Mullen and the chiefs. But we're all realistic. The history of this conflict has demonstrated that we must always be prepared for the unpredictable and that we must be extremely cautious with our every step."

President Bush announced on Thursday that after the currently scheduled troop withdrawals are completed in July, he would give his top commander in Iraq 45 days to evaluate the effects of the drawdown. That would be followed by an indefinite period to reassess U.S. troop strength in Iraq and determine the timing of additional troop reductions.