Defense Secretary Robert M. Gates held out the prospect today that the United States might be able to start withdrawing some of its forces from Iraq by the end of this year if a new Baghdad security plan is successful. But he said the plan "is not the last chance" in Iraq and that he is considering possible alternatives if the buildup fails to quell rampant sectarian violence.

Testifying before the Senate Armed Services Committee on the administration's latest budget request for the Defense Department, Gates also announced that President Bush has decided to create a new military command for Africa to oversee various missions on the continent, including possible military operations.

During the hearing, the committee voted to approve the nominations of Gen. George W. Casey Jr. to be Army chief of staff and Adm. William J. Fallon to take over as head of the U.S. Central Command. The nominations now go to the Senate floor for a confirmation vote by the full Senate.

In response to questions, Gates said the proposed budget contains no money for Bush's planned troop surge in Iraq after Oct. 1, the start of the 2008 fiscal year. He said this is because the reinforcement is anticipated to last for months, "not 18 months to two years."

Bush's plan, announced Jan. 10, calls for sending 21,500 additional U.S. troops to Iraq, with five Army brigades totaling about 17,500 soldiers deployed to Baghdad and one Marine contingent of about 4,000 troops reinforcing the volatile western province of Anbar.

Under questioning from the committee chairman, Sen. Carl M. Levin (D-Mich.), Gates said the administration could decide to withhold the deployment of some of those troops if the Iraqi government fails to meet its commitments on the Baghdad security plan, notably on the number of Iraqi troops committed to it and on its pledge to refrain from political interference in military operations.

"We will have a continuing evaluation going on in terms of the Iraqis' performance," Gates said.

"We clearly are hoping it will succeed, planning for it to succeed, allocating the resources for it to succeed -- but I would tell you that I think I would be irresponsible if I weren't thinking about what the alternatives might be if that didn't happen," Gates told Sen. John W. Warner (R-Va.). "But we, at this point, are planning for its success."

Pressed by Sen. Robert C. Byrd (D-W.Va.) on how much longer the U.S. military involvement in Iraq would continue, Gates said, "It's hard to make any kind of a real prediction, especially where our adversaries have a vote. But it seems to me that if the plan to quiet Baghdad is successful and the Iraqis step up . . . and successfully assume the leadership in trying to establish order and then . . . further carrying out their political reconciliation process, I would hope that we would be able to begin drawing down our troops later this year."

Gates added, "I think that there's very likely to be at least some American presence in Iraq for a number of years. . . . But my hope would be that over the longer term that it would be a fraction of what we have there now."

Asked by Sen. Evan Bayh (D-Ind.) what the consequences would be if the Iraqis fail to meet their commitments, the defense chief said, "First, obviously, we're going to try and persuade them to do what they promised to do. But then there is always the potential of withholding assistance or of changing our approach over there in terms of how we interact with that government."

"The success of this strategy is dependent entirely on the Iraqis' willingness to fulfill the commitments they made to us," he said.

Sen. Hillary Rodham Clinton (D-N.Y.) accused the administration of giving the Iraqi government "a blank check" and then allowing it to ignore U.S. demands for progress on national reconciliation and other political and economic issues. She called for an end to what she described as the U.S. carrot-and-stick approach toward the government of Prime Minister Nouri al-Maliki, saying that "I'm not sure carrots are part of the diet" at this point.

Gates appeared at the hearing with Marine Gen. Peter Pace, chairman of the Joint Chiefs of Staff, and Tina W. Jonas, the Defense Department comptroller.

The hearing was interrupted several times by hecklers protesting the war in Iraq.

In the House, meanwhile, Democratic leaders said they plan to hold a vote next week on a nonbinding resolution opposing the troop-surge plan. House Majority Leader Steny H. Hoyer (D-Md.) said the vote would likely take place after three days of debate on the resolution starting Feb. 13. Senate Republicans succeeded yesterday in blocking debate on a similar resolution, and Majority Leader Harry M. Reid (D-Nev.) said no new negotiations are underway.

The votes to recommend the confirmation of Casey and Fallon were taken during a break in the hearing when the Armed Services Committee attained a quorum.

Fallon, who succeeds Army Gen. John P. Abizaid as commander of U.S. forces in the Middle East, was approved by a unanimous vote of the committee members present. He has spent the past two years as the senior U.S. commander in the Pacific and is expected to take up his new post by next month.

The nomination of Casey, the outgoing commander of U.S. forces in Iraq, was approved 14 to 3, with three Republican senators -- John McCain (Ariz.), John Ensign (Nev.) and Saxby Chambliss (Ga) -- voting no. Several other senators were not present and their votes were to be recorded later. McCain sharply criticized Casey in a hearing last week, saying he had given "unrealistically rosy" assessments of the situation in Iraq.

At today's hearing, McCain questioned Casey's depiction of the U.S. military's Iraq strategy as successful, when other senior military officers, including Pace, have stated that the United States is not currently winning in Iraq. Pace said he could not speak for Casey, but he noted that Casey was the first officer to go to the Joint Chiefs and tell them the security situation in Iraq would not permit him to carry out a planned drawdown of U.S. troops in 2006.

"I still find it hard to reconcile General Casey's continued assertion that the previous strategy was working, in contravention to the view of most any observer," McCain said. "We're now going to put him in charge of the training and equipping responsibilities for this new strategy when, obviously, he believes the old one was just fine."

If, as expected, he is confirmed by the full Senate, Casey, who formerly served as the Army's vice chief of staff, will replace Gen. Peter J. Schoomaker as Army chief of Staff. Schoomaker was called out of retirement in August 2003 by then-Defense Secretary Donald H. Rumsfeld to lead the Army.

In announcing the new command for Africa, Gates told the committee, "The president has decided to stand up a new unified combatant command, Africa Command, to oversee security, cooperation, building partnership capability, defense support to nonmilitary missions, and, if directed, military operations on the African continent."

He said the command "will enable us to have a more effective and integrated approach than the current arrangement of dividing Africa between Central Command and European Command, an outdated arrangement left over from the Cold War." He pledged to work closely with Congress and U.S. allies in Europe and Africa to establish the new command.

Gates provided no details on the new command at the hearing, and a military official familiar with planning for it told the Associated Press that decisions on personnel, location of the headquarters and other details are not yet complete. A transition team soon will begin working from facilities in Stuttgart, Germany, the European Command headquarters, but officials ultimately want the headquarters somewhere in Africa, AP reported.

The new command will include islands around Africa and all nations on the continent except for Egypt, which will stay in Central Command, the agency quoted the defense official as saying. Bush authorized the Pentagon to set up the command no later than September 2008.

Levin, the new chairman of the Armed Services Committee opened the hearing with a statement criticizing the cost in lives and money of Bush's Iraq war policy. The proposed Pentagon budget for fiscal 2008, including the cost of operations in Iraq and Afghanistan, is now 60 percent larger than it was in 2001 when Bush took office, Levin said. "Yet it is difficult to see how we as a nation are more secure," he said.

"Nowhere are the costs of this administration's policies more vivid than in the budget request before us today for operations in Iraq," Levin added. "Our nation has already made an investment of over $330 billion and a sacrifice of over 3,000 lives lost and 22,000 wounded in Iraq. But this has not brought us the secure and stable Iraq that we all wish to see."

The administration wants to spend "an additional $174 billion in a more robust version of that same Iraq policy for the balance of 2007 and 2008," Levin said. "That would bring the total invested in military costs in Iraq to $505 billion by the end of 2008 fiscal year."

Gates acknowledged "sticker shock" at the combined price tags of the administration's budget requests for the armed forces, including U.S. military operations around the world. He put those costs at "more than $700 billion."

But he noted that the projected spending on defense this year comes to about 4 percent of U.S. gross domestic product, which he said is a smaller percentage of GDP than when he left his position as CIA director 14 years ago at the end of the Cold War. He said it was also "a significantly smaller percentage than during previous times of war, such as Vietnam and Korea," even though the world has become "more complicated and arguably more dangerous" since then.

Gates and Pace took issue with a report by the nonpartisan Congressional Budget Office last week estimating that the surge in Iraq could end up sending a force twice the size of the planned 21,500 when support troops are included. The CBO estimated that the total number of personnel could range from about 35,000 to 48,000, with costs ranging from $9 billion to $13 billion for a four-month deployment to $20 billion to $27 billion for a 12-month deployment.

The two officials said the number of extra support personnel who could be sent to Iraq would not be more than 10 to 15 percent of the 21,500, meaning that the total additional force could approach 25,000.

Pace said most of the needed support personnel are already in Iraq.

The Marine general also told the committee that the number of roadside bombs that have been emplaced by insurgents in Iraq has doubled over the past year, but that U.S. casualties have fairly constant because of improved tactics and equipment to counter them. The bombs, known as improvised explosive devices, or IEDs, account for about 70 percent of U.S. casualties in Iraq,

"The basic material for an IED is ammunition," Pace said. "So far, we have cleared 430,000 tons of ammunition from over 15,000 sites in Iraq. The amount of ammunition available is incredible. You add that, then, to the new explosively formed projectiles, which are a much more deadly form that are coming into Iraq from Iran, and the combination has maintained the level of casualties despite the fact that we have been more effective against each explosion."