WASHINGTON (CNN) -- The top U.S. officials in Iraq told Senate committees Tuesday that Iranian agents and weapons are fueling the ongoing strife there and that further U.S. troop withdrawals will have to wait.
Gen. David Petraeus tells Congress on Tuesday that conditions on the ground must dictate troop levels.
Gen. David Petraeus, the commander of U.S. troops in Iraq, and U.S. Ambassador to Iraq Ryan Crocker spent about nine hours giving the Senate Armed Services and Foreign Relations committees their assessments of the 5-year-old war.
Petraeus said that in the seven months since their last appearance before Congress, U.S. and Iraqi forces have made progress toward tamping down the violence but that progress was "fragile" and "reversible."
Although the last of the additional U.S. combat brigades dispatched in 2007 is scheduled to leave in June, Petraeus said he would recommend against further withdrawals for at least 45 days.
"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 far and sacrifice so much to achieve."
And as expected, Petraeus repeated his position that future troop levels be based on conditions on the ground.
"War is not a linear phenomenon," he said. "It's a calculus, not arithmetic." Watch a report on Petraeus' facts »
Meanwhile, he said, Iran is "funding, training, arming and directing" Shiite Muslim militias known as "special groups." He said the recent fighting between Iraqi government troops and militias in southern Iraq highlighted that involvement.
"Unchecked, the special groups pose the greatest long-term threat to the viability of a democratic Iraq," Petraeus told the Senate Armed Services Committee in the first of two hearings on the war in Iraq.
And Crocker said Tehran has become a "malign influence" on the war. But he said the reaction to the recent upswing in violence has the potential to turn Iraqi public opinion against Iranian influence.
"This would be an excellent time for them to reassess what is ultimately in their own long-term interest," he said.
Crocker and Petraeus spent more than nine hours answering questions from members of the Armed Services and Foreign Relations committees in the first of two days of hearings on the progress of the war, which has claimed more than 4,000 U.S. lives and cost an estimated $600 billion since 2003. It is now widely unpopular at home, with a CNN poll in March showing about two-thirds of the country opposes the conflict.
Army leaders have warned the war has left the service "out of balance," with troops strained by extended deployments and equipment worn by exposure to harsh desert conditions. President Bush is expected to announce Thursday that the military will reduce the length of troops' combat tours from 15 months to 12 months.
Opening Tuesday's Senate hearings, the Armed Services 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 Bush of "ignoring the view of his own military leaders." Watch Levin grill the general »
Levin further said that Iraqis were not allocating enough of their own money and resources "to take responsibility for their country's future."
Later, Sen. Joseph Biden, the Foreign Relations Committee's chairman, said continued U.S. involvement was "unsustainable."
"We have gone from drowning to treading water," said Biden, D-Delaware, who mounted an unsuccessful presidential campaign this year. "We are still spending $3 billion every week, and we are still losing ... 30 to 40 American lives every month. We can't keep treading water without exhausting ourselves, and that is what the president seems to be asking us to do."
And the committee's ranking Republican, Indiana Sen. Richard Lugar, questioned how the military gains could be converted to a settlement that would allow U.S. withdrawals he said would be forced by strains on the U.S. armed services.
"Simply appealing for more time to make progress is insufficient," Lugar said.
The testimony came after two weeks of fighting among Shiite factions in southern Iraq and a continued push against Islamic jihadists loyal to al Qaeda in the north.
Iran has ties to several players in the power struggle among Shiite factions in the south: The leading parties in Iraqi Prime Minister Nuri al-Maliki's ruling coalition have longstanding ties to Tehran, which backed Iraqi Shiite opposition forces during Hussein's rule. And a weeklong operation against Shiite militias loyal to Iraqi cleric Muqtada al-Sadr in southern Iraq, a battle on which al-Maliki staked his prestige, ended only after negotiations in Iran among Iraqi lawmakers, Iranian leaders and al-Sadr's representatives.
Petraeus told senators the Iraqi government's operation in Basra "could have been better planned, and the preparation could have been better." He said that once the forces got into the southern Iraqi city, "they ended up going into action more quickly than anticipated" and al-Maliki didn't follow his advice to move at a more careful pace. See military charts showing violence levels »
The hearings also exposed Petraeus and Crocker to questioning from all three of the major presidential candidates: Sen. John McCain, the presumptive Republican nominee, and Sens. Hillary Clinton and Barack Obama, the Democratic candidates. Watch panelists discuss Iraq war's impact on next president »
McCain, the top Republican on the Armed Services Committee and a leading advocate of last year's troop increase, said the United States is no longer "staring into the abyss of defeat" as a result. Watch McCain make a case for staying in Iraq »
"Success, the establishment of a peaceful, democratic state, the defeat of terrorism -- this success is within reach," he said. "Congress must not choose to lose in Iraq. We must choose to succeed."
But Clinton responded that it would be "irresponsible" to continue a failed policy in Iraq. She said it is "time to begin an orderly process of withdrawing our troops" from Iraq in order to focus on Afghanistan and other U.S. interests. Watch more of Clinton's comments »
And Obama, a member of the Foreign Relations Committee, questioned whether the conditions set by U.S. commanders for withdrawal would lead to a war that could last 20 to 30 years. He called the invasion of Iraq a "massive strategic blunder" that allowed al Qaeda and Iran to spread their influence into Iraq, and said the United States should pressure Iraqi officials to settle the war by threatening to leave. Watch why Petraeus says al Qaeda's still a threat »
"Nobody's asking for a precipitous withdrawal, but I do think it has to be a measured, but increased, pressure and a diplomatic surge that includes Iran," Obama said. "Because if Maliki can tolerate normal neighbor-to-neighbor relations in Iran, then we should be talking to them as well. I do not believe we're going to be able to stabilize the situation without them."
Iraq's ambassador to the United States said Tuesday that the United States has to keep its forces in his country unless it wants Iran to have a free hand in Iraq.
"It is not something you can get out of so easily," Ambassador Samir Sumaidaie said after a speech at the Center for Strategic and International Studies, a Washington think tank. "This is the wrong time, unless you want to hand the country on a plate to Iran."
He said American forces "have to leave in a responsible manner. We want them to leave. Let's be clear." E-mail to a friend
CNN's Charley Keyes contributed to this report.