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.