Iranian support for militias in Iraq has grown, top U.S. defense leaders said Friday, asserting that recent battles in Basra gave the Iraqis an eye-opening view of Iran's increased negative role there.

Defense Secretary Robert Gates said the U.S. will be as aggressive as possible to counter that increase, adding that the Iraqis "are in a position themselves to bring some pressures to bear on Iran."

Speaking after he and his commanders spent three days on Capitol Hill mapping out progress in Iraq, Gates also acknowledged that future troop withdrawals will go more slowly than he had initially hoped last year.

"I think that the process has gone a little slower," Gates told a Pentagon news conference Friday. He said that plans -- endorsed by Bush on Thursday -- to halt troop withdrawals at least until mid-September would make it a "real challenge" to pull out five additional brigades by the end of the year.

Last year Gates said he was holding out hope that the U.S. presence in Iraq could drop to about 10 brigades -- or roughly 100,000 troops by the end of this year. On Thursday, he told senators he had abandoned that hope.

Iran's role has been one of the complicating factors.

"I think that there is some sense of an increased level of supply of (Iranian) weapons and support to these groups," said Gates, referring to what the military has termed "special groups" of Shiite militants. "But whether it's a dramatic increase over recent weeks, I just don't know."

He and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said recent clashes between Iraqi security forces and Shiite militias in Basra highlighted the increase in Iranian support.

"I think the Iraqi government now has a clearer view of the malign impact of Iran's activities inside Iraq," said Gates. "I think they have had what I would call a growing understanding of that negative Iranian role. But I think what they encountered in Basra was a real eye-opener for them."

Late last year, military commanders suggested Iran may be slowing the flow of illegal weapons across the border into Iraq. And Tehran recently helped broker a truce between the Iraqi government and radical Shiite cleric Muqtada al-Sadr, whose Mahdi Army is battling U.S. and Iraqi troops in Baghdad.

On Friday, Gates and Mullen discounted that suspected decrease in weapons flow.

In related comments, Ryan Crocker, the U.S. ambassador in Iraq, said Friday that the Bush administration wants to test whether Iran is really interested in talking to the United States about Iraq.

He said that even though Iran has backed out of recent appointments to meet with him for such discussions, he's ready and waiting for the meeting. Crocker said the administration thinks it's worthwhile to make the case to Iran that it should butt out of Iraq's internal affairs.

Gates, meanwhile, said Friday that he doubts Muqtada al-Sadr 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.

Asked if an arrest was possible, Gates said, "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."

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

Separately, Gen. David Petraeus, the top U.S. commander in Iraq, told a small group of reporters at the Pentagon that al-Sadr's organization has to be reckoned with.

"It is a movement that must be addressed and, to varying degrees, accommodated," Petraeus said.

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.

Mullen, 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."

While Gates said troop withdrawals will go more slowly than he initially wanted, he reiterated that he is "confident that we will have a lower number of troops" in Iraq next year.

"I'm not saying when in 2009, but I believe we will have a lower number of troops in Iraq in 2009," he said, adding that he also still 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 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.