Top military officials warned Congress yesterday that a quick withdrawal of American troops from Iraq could cause the country to spiral into even deeper chaos that would destabilize the region and create a safe haven for Al Qaeda.

The Bush administration also signaled a new flexibility in its policy toward Iran, offering the possibility of direct Iranian-American talks that some in the administration had previously warned against.

“Failure to stabilize Iraq could increase Iranian aggressiveness and could embolden Al Qaeda’s ideology,” the commander of American troops in Iraq, General John Abizaid, said during parallel hearings held by Senate and House committees yesterday.

“I believe the coalition forces right now are the element that is keeping Iraq together,” the director of the Defense Intelligence Agency, Major General Michael Maples, said. “The impact of removal of the forces, I think, will lead us to a greater level of violence, perhaps, in Iraq than what we’re seeing now.”

The State Department’s top official for Iraq policy, David Satterfield, said the White House is prepared to deal directly with the Iran in an effort to stem that country’s support for militias that are undermining the fledgling Iraqi regime.

“We are prepared in principle to discuss Iranian activities in Iraq,” Mr. Satterfield said. “The timing of such a direct dialogue is one we still have under review.”

The director of central intelligence, General Michael Hayden, said Iran has now become a major player in Iraqi affairs and is exhibiting an attitude of “almost dangerous triumphalism.”

“The Iranian hand appears to be powerful,” the general told the Senate Armed Services Committee. “It appears to be growing and Iranian ambitions in Iraq seem to be expanding.”

General Hayden declined to get into details in public about the Iranian activities, but he said they are supplying Shiite militias in Iraq that have mounted attacks on American troops. “The provision to them of capabilities that have been used against the coalition has been quite striking,” General Hayden said. “I have come late to this conclusion but I have all the zeal of a convert as to the ill effect the Iranians are having on the situation in Iraq.”

The CIA director acknowledged that the decision to talk to the Iranians about Iraq was “fraught with other policy considerations,” chiefly concerns about the Iranian nuclear program. The administration has steadfastly rejected direct discussions with Iran on that subject, though talks have proceeded through the British and other intermediaries.

“I’d be real skeptical about anything good coming out of talking with the Iranians,” Senator Lieberman of Connecticut, an independent, said.

General Abizaid faced sharp challenges and questions from senators, many of whom, coincidentally or not, are contemplating runs for the White House in 2008.

Senator McCain of Arizona, who is expected to announce today that he is setting up a committee to explore a bid for the Republican nomination, pressed the general on his claim that he has enough American troops in Iraq to carry out the mission. Under questioning, General Abizaid conceded there are not enough American troops in a troubled area outside Baghdad, Al Anbar province, to control insurgent violence there. “Al Anbar province is not under control,” the general said, adding that a decision has been made to try first to clamp down on violence in the capital.

“It might be well to get both Baghdad and Al Anbar under control before we have another battle of Fallujah,” Mr. McCain said.

“I don’t believe you can have a main effort everywhere,” General Abizaid said.

“I don’t get that tactic, General,” Mr. McCain said, accusing him of “advocating the status quo” and ignoring the message of last week’s election.

Mr. McCain has proposed boosting American troop levels in Iraq by about 20,000 in order to gain a handle on the violence, but the general said more Americans might slow Iraqi progress.

“We want the Iraqis to do more. It’s easy for the Iraqis to rely upon us to do this work. I believe that more American forces prevent the Iraqis from doing more, for taking more responsibility for their own future,” General Abizaid said.

The general’s stance was a curious one, since his logic was quite similar to that of some of the most ardent proponents of a quick American withdrawal, such as Rep. John Murtha of Pennsylvania and Senator Kerry of Massachusetts, both Democrats. They have argued that the departure of the Americans is the only thing that will prompt Iraqis to get serious about the political compromises needed to establish a functional government.

General Abizaid also made a candid admission that the Pentagon would be hard pressed to boost the number of American troops assigned to Iraq beyond the current level of 141,000 for anything more than a brief period. “We can put in 20,000 more Americans tomorrow and achieve a temporary effect. The ability to sustain that commitment is simply not something that we have right now with the size of the Army and the Marine Corps,” the general said.

A Democratic presidential prospect for 2008, Senator Clinton of New York, also lit into the administration witnesses. “Hope is not a strategy. Hortatory talk about what the Iraqi government must do is getting old,” Mrs. Clinton said. “I have heard over and over again, ‘The government must do this. The Army must do that.’ Nobody disagrees with that. The brutal fact is, it is not happening.”

Mrs. Clinton raised the possibility of partitioning Iraq along sectarian lines, though she stopped short of endorsing the idea, a more limited version of which has been proposed by Senator Biden, a Democrat of Delaware.

Administration officials rejected partition outright, saying it was impractical because there are not clear geographic boundaries separating sectarian groups.

“Partition in Iraq could only be achieved at an expense of human suffering and bloodshed and forced dislocation that would be both profound and wholly unacceptable, I believe, to the American people. It is wholly unacceptable to this administration,” Mr. Satterfield said. “It’s not a practical option. It is not a moral option.”

Some lawmakers were skeptical about General Abizaid’s candor. Senator Dayton, a Democrat of Minnesota, said recent books about the Iraq war, including “State of Denial” by Bob Woodward and “Fiasco” by Thomas Ricks, led him to conclude that there were “contradictions” between what the general told Congress at earlier hearings and what he was saying to others behind the scenes. General Abizaid insisted he had been honest with the committee.

Others suggested the general might be slow on the uptake. After he made a comment about the need to transfer security responsibilities to Iraqis from American forces, Rep. Ellen Tauscher, a Democrat of California, replied, “My 15-year-old has known that for two years. … We’re never going to have enough Iraqis who are going to fight and die for a feckless government.”

“I believe we must stick with them until they show us they can’t do it,” General Abizaid told the Senate earlier in the day. “Those among us who fight bet on the Iraqis and as long as they’re confident, I’m confident.”

Mr. Satterfield occasionally slipped into diplo-speak that was hard to follow. In response to a question about the viability of a political settlement in Iraq, he referred to “a sufficient degree of minimal convergence” among the various groups in the country.