"It's sad, it's a sad day when you lose life," Mr. Bush said at the White House upon his return from a weekend in Camp David.
"It doesn't matter whether it's in a chopper crash or an IED (improvised explosive device), the loss of life is sad."
The military was investigating whether insurgent groundfire caused the crash of two U.S. helicopters, killing 17 American soldiers. It was the worst single loss of U.S. life since the start of the Iraq war.
Asked how the United States would ensure security in Iraq once the American occupation has ended, Mr. Bush said that could only be determined by the progress in making the country safe and defeating insurgents.
"It depends on what's taking place on the ground," he said. "We'll adjust our troop levels according to the security situation in Iraq."
The president also reacted to an audiotape purportedly made by Saddam Hussein in which the deposed leader tells Iraqis to step up their resistance to the U.S.-led occupation.
Mr. Bush said he had not seen specifics of the recording. But, Mr. Bush said, such threats would not cause the United States to leave Iraq.
"I suspect it's same old stuff. It's propaganda," Mr. Bush said "We're not leaving until the job is done, pure and simple. ... I'm sure he'd like to see us leave, if, in fact, it's his voice."
The speaker on the tape broadcast Sunday says the United States and its allies misjudged the difficulty of occupying Iraq.
In other developments:
From Baghdad, the Iraqi capital, America's chief postwar administrator in Iraq said the United States will help write an interim Iraqi constitution that embodies American values and will lead to the creation of a new government.
"We will write into that constitution exactly the kinds of guarantees that were not in Saddam's constitution.," L. Paul Bremer told ABC's "This Week."
"We'll have a bill of rights. We'll recognize equality for all citizens. We'll recognize an independent judiciary. We'll talk about a federal government," Bremer said.
"All of these things will be in the interim constitution which will also provide in a limited time, probably two years, for a permanent constitution to be written that also embodies those American values," he said.
The Senate's top Democrat, Tom Daschle of South Dakota, expressed concern about what he said was the deteriorating situation in Iraq today.
"I don't know that we can say we're losing. ... I'm not sure we're winning the hearts and minds of the Iraqi people today, and that's of a real concern," he told "Fox News Sunday."
"I think the president needs, first and foremost, a plan for success, not an exit strategy. I think they're putting too much emphasis on exit and not enough emphasis on success," he said.
Mindful of the helicopter crash and the escalation of guerrilla warfare, Mr. Bush said he had spent part of Sunday "in prayer for our service men and women who are in harm's way."
"The sacrifice that our folks are making in Iraq will serve our nation's interest in the short term and the long term," he said. "It's best we defeat the terrorists in Iraq so we don't have to defeat them here."
The president praised a White House-endorsed agreement, announced by the Iraqi Governing Council on Saturday, to form a provisional government by June.
"It's a tough week, but we made progress for a sovereign and free Iraq," he said. "On the one hand the politics is moving on, on the other we're going to stay tough and deal with the terrorists."
Bremer said that while the U.S.-led occupation will end with the handover to the council, said the presence of coalition forces will not.
"Our presence here will change from an occupation to an invited presence," he said. "I'm sure the Iraqi government is going to want to have coalition forces here for its own security for some time to come."
Defense Secretary Donald H. Rumsfeld, traveling in Asia, said the United States continues to plan to rotate a new contingent of troops into Iraq next year, with no final pullout date set yet. Accelerating the political process toward Iraqi self-rule will not affect military planning, he said.