"None of this will be rapid," Lt. Gen. David Petraeus told the Senate Armed Services Committee. "The way ahead will be neither quick nor easy. There undoubtedly will be tough days."
Many in Congress, including some Republicans, oppose Mr. Bush's plan, which would send an extra 21,500 U.S. troops to Iraq as part of a revised strategy for quelling sectarian violence in Baghdad and stabilizing the country. Before the buildup began in recent days, there were 132,000 U.S. troops in Iraq.
Mr. Bush nominated Petraeus to replace Army Gen. George Casey as the senior American commander in Iraq. Petraeus told the committee he had spoken to Casey in recent days and that Casey said he favored Mr. Bush's troop buildup.
According to a recent CBS News poll, two-thirds of Americans remain opposed to the president's plan for sending additional U.S. troops to Iraq. And he should seek congressional approval for the troop increase.
Sen. Carl Levin, a Democrat, chairman of the committee and a leading critic of Mr. Bush's Iraq policy, pressed Petraeus on whether the flow of additional U.S. troops could be halted in midstream if the Iraqi government failed to meet its commitment to provide thousands more Iraqi troops.
"It could," Petraeus replied. Earlier he said there were no "specific conditions" the Iraqis must meet in order to keep the flow of U.S. forces moving. The last of five additional U.S. brigades are scheduled to arrive in the Iraqi capital in May; the first got there just days ago.
Petraeus said that in the event the Iraqis did not meet their commitments, he would consult with Defense Secretary Robert Gates on how to respond.
In his opening statement, Petraeus, 54, painted a grim picture of conditions in Iraq.
"The situation in Iraq is dire," he said. "The stakes are high. There are no easy choices. The way ahead will be very hard. ... But hard is not hopeless."
Famous for challenging 20-year-old solders to push-up contests and winning, Petraeus has a nearly perfect resume for the position of commander of multi-national forces in Iraq, reports CBS News national security correspondent David Martin. He commanded the 101st Airborne in the initial invasion and was in charge of training Iraqi security forces.
Petraeus is considered a shoo-in to win Senate confirmation. Devoted early in the war to trying to win the hearts and minds of Iraqis, Petraeus later wrote the Pentagon manual on how to tackle insurgencies. He also previously supported expanding U.S. forces in the region.
Sen. John McCain, the top Republican on the committee and a leading proponent of Mr. Bush's troop buildup plan, asked Petraeus how long he thought the U.S. buildup could be sustained.
"I am keenly aware of the strain" on the Army and Marine Corps, Petraeus said, adding that he welcomes Mr. Bush's proposal to increase the size of the land forces over the coming five years.
Asked by McCain how soon he thought he would know whether the new strategy was working, Petraeus said, "We would have indicators at the least during the late summer." As currently planned, he said, the last of the five additional U.S. Army brigades would be ready to fight in Baghdad by the end of May.
Several committee members noted that Petraeus recently oversaw the writing of a new Army manual on how to counter an insurgency. Sen. Edward Kennedy asked him why an extra 21,500 troops would make a significant difference.