- NEW: Sens. John McCain, Hillary Clinton disagree over level of progress in Iraq
- Ambassador, general warn that Iran could pose greatest threat to Iraq
- Hearing twice interrupted by protesters, one chanting, "Bring them home"
- U.S. ambassador says Iraqi parliament working on national reconciliation
WASHINGTON (CNN) -- Iraq is making "fragile but reversible" progress on security, but it's too early to set dates to pull out all U.S. troops, the top U.S. military commander in Iraq told Congress on Tuesday.
Gen. David Petraeus said the number of troops should return to "pre-surge" levels this summer, but the military should gauge conditions before making further decisions.
After the 20,000 troops sent during last year's surge are withdrawn, by July, the military should wait 45 days before deciding on more reductions, Petraeus told the Senate Armed Services Committee.
"This approach does not allow establishment of a set withdrawal timetable," he said. "However, it does provide the flexibility those of us on the ground need to preserve the still fragile security gains our troopers have fought so hard and sacrificed so much to achieve."
There has been "significant but uneven progress," Petraeus said, adding that recent violence shows the progress is "fragile but reversible."
"The situation in certain areas is still unsatisfactory and innumerable challenges remain," he said.
Petraeus said the surge of U.S. troops last year and the incorporation of Iraqi citizens' security groups have yielded results. Both efforts have helped reduce "the areas where al Qaeda enjoys support." Watch entire hearing live on CNN.com
"Iraq has also conducted a surge, adding well over 100,000 additional soldiers and police to the ranks of its security forces in 2007 and slowly increasing its capability to deploy and employ these forces," Petraeus said.
Recent military operations in the southern city of Basra demonstrate that Iraqi forces can do things today that would have been impossible a year ago, the general said.
Ryan Crocker, the U.S. ambassador to Iraq, who testified alongside Petraeus, said Iraqis have shown progress on the political front as well, with parliament -- previously paralyzed by infighting -- passing legislation that advances "reconciliation and nation building."
Both men warned that with al Qaeda in Iraq retreating -- though not yet defeated -- Iran is the most likely force to derail the country's tenuous stability.
The flow of insurgents through Syria -- while reduced to a degree -- also exacerbates problems in Iraq, as do "insufficient Iraqi governmental capacity, lingering sectarian mistrust and corruption," Petraeus said.
The Islamic republic is "funding, training, arming and directing the so-called special groups," which left unchecked, "pose the greatest long-term threat to the viability of a democratic Iraq," he said.
While vowing to "aggressively uproot and destroy" these forces, Crocker also said diplomacy was essential: "We support constructive relations between Iran and Iraq and are participating in a tripartite process to discuss the security situation in Iraq. Iran has a choice to make."
Conceding the many convoluted situations plaguing the war-ravaged nation, Crocker said, "Mr. Chairman, almost everything about Iraq is difficult."
The hearing was twice interrupted by protesters, one of whom was escorted out of the Senate chamber as he chanted, "Bring them home."
Petraeus and Crocker are scheduled to answer questions from key senators Tuesday and from House members Wednesday.
Opening Tuesday's Senate hearing, the committee chairman said the United States must come up with a timeline for ending its involvement in Iraq.
"Our current open-ended commitment is an invitation to continuing dependency," Sen. Carl Levin, D-Michigan, said, before accusing President Bush of "ignoring the view of his own military leaders."
Levin further said that Iraqis were not allocating enough of their own money and resources "to take responsibility for their country's future."
In later remarks, Levin criticized Petraeus, saying the general's proposal to wait on determining troop levels as a "plan which has no end to it."
Sen. John McCain, the presumed GOP presidential candidate and the committee's ranking member, acknowledged that Iraqi officials need to take responsibility for their country.
However, despite a recent upsurge in fighting in Iraq, McCain said there were signs of progress there.
"We are no longer staring into the abyss of defeat," McCain said, adding that he has no desire to keep U.S. troops in Iraq "a minute longer than necessary."
Sen. Hillary Clinton, a committee member and contender for the Democratic presidential nomination, flatly told Petraeus that it's "time to begin an orderly process of withdrawing our troops," so the U.S. can focus on Afghanistan and other interests.
Citing what she called a "lack of political progress over the past six months" and lackluster Iraqi-led military offensives -- particularly in southern Iraq -- Clinton slammed the U.S. policy in Iraq.
"It might well be irresponsible to continue the policy that has not produced results that have been promised time and time again," she said.
Clinton's Democratic rival, Sen. Barack Obama, was scheduled to ask Crocker and Petraeus questions later Tuesday when they appear before the Senate Foreign Relations Committee.
Last month, Petraeus said the military was in a "better place in terms of security, and even in terms of political progress by the Iraqis, than we were, say, a year ago. Although we are always quick to note that the progress is tenuous and that it is reversible and that there are innumerable challenges out there."
At the end of March, a high-profile week of fighting erupted in Basra between Iraqi government forces and Shiite militias.The clashes followed months of relative calm in which violence seemed to be declining. Military results were inconclusive.
The battle ended only after Iraqi Shiite lawmakers traveled to Iran to negotiate with Iranian leaders and Iraqi Shiite cleric Muqtada al-Sadr, who commands the loyalty of the largest militia in Iraq.
White House spokesman Tony Fratto admitted Monday the Basra operation was not "an overall success" for Iraqi government forces.
After the surge ends in July, there will be 15 U.S. combat brigade teams in Iraq, and roughly 140,000 American troops.
In September, Defense Secretary Robert Gates floated the prospect of pulling five additional brigades out of Iraq by 2009.
Last testifying with Crocker in September, Petraeus said he would be hard-pressed to recommend maintaining the surge of U.S. troops past this spring if conditions in Iraq did not improve in the interim.
Since then, however, Petraeus has requested a "pause" in troop withdrawals following the end of the surge.
Officials have said privately it's hoped that one or two brigades could leave before Christmas.
Such a reduction may be possible because of plans to shorten Army combat-zone tours from 15 months to 12 months as of this summer. Bush is expected to announce that change Thursday.
All AboutRyan Crocker • Muqtada al-Sadr • Iraq • David Petraeus