WASHINGTON — Marking a grim milestone, a determined President Bush declared Monday the lives of 4,000 U.S. military men and women who have died in Iraq "were not lost in vain." The White House signaled anew that additional troops won't be pulled out soon.

A roadside bomb in Baghdad killed four U.S. soldiers Sunday night, pushing the death toll to 4,000.

That number pales compared with those of other lengthy U.S. wars, but it is much higher than many Americans, including Bush, ever expected after the swift U.S. invasion of Iraq five years ago.

Bush proclaimed the end of major combat operations in Iraq in May 2003. Almost all of the U.S. deaths there have happened since then.

"One day people will look back at this moment in history and say, 'Thank God there were courageous people willing to serve, because they laid the foundations for peace for generations to come,'" Bush said after a State Department briefing about long-term diplomacy efforts.

"I have vowed in the past, and I will vow so long as I'm president, to make sure that those lives were not lost in vain _ that, in fact, there is an outcome that will merit the sacrifice," Bush said.

The news of 4,000 dead in Iraq came the week after the war rolled into its sixth year, dominating most of Bush's presidency. Almost 30,000 U.S. service members have been wounded in the war.

Early in April, Bush is expected to announce the next steps in the war, and he is likely to embrace a pause in any troop withdrawals beyond those scheduled to end this July.

Democrats in Congress and on the presidential campaign trail continue to push for a faster end to war. But Bush still has the upper hand for 10 months.

"Americans are asking how much longer must our troops continue to sacrifice for the sake of an Iraqi government that is unwilling or unable to secure its own future," said House Speaker Nancy Pelosi. She said the cost to the U.S. reputation is immense, and the threat to the economy at home is unacceptable.

Senate Majority Leader Harry Reid said the death toll is a reminder that the nation must get out of an "endless civil war and make America more secure."

The White House was careful in its reaction to the milestone, calling it a sober moment but emphasizing that deaths are grieved no matter what the number. Bush said people are praying for the families of those killed whether they were among the first or the most recent casualties.

The number killed in Iraq is far less than in other modern American wars.

In Vietnam, the U.S. lost more than 58,000 troops, passing the 4,000 mark in 1966 as deaths rose quickly along with escalating American involvement.

Grim milestones such as new death toll often go unremarked by Bush. But he chose on this occasion to note the losses, albeit briefly and without taking questions from reporters.

As always, his message was determination.

"Our strategy going forward will be aimed at making sure that we achieve victory and, therefore, America becomes more secure," he said. Just last week, Bush said the high cost in lives and treasure was necessary to halt the spread of terrorism and keep Iraq out of chaos.

The White House said Bush is likely to embrace an expected recommendation from his top commander in Iraq, Gen. David Petraeus, for a halt in troop withdrawals in July. During the pause, Petraeus and other senior commanders would make yet another assessment of conditions in Iraq, possibly in September, before recommending any specific troop reductions for the final months of 2008.

One factor arguing against a quick resumption in troop cuts after July is the likelihood that Iraq will hold provincial elections in October, an event that probably will require heightened security.

"It's now a question of how much time do we pause for," said Max Boot, a conservative expert on national security and a senior fellow at the Council on Foreign Relations. "Petraeus has been pushing for it, and I think the president will certainly give it to him. He has a pretty good hand right now just from the political calculus _ he can make a decision on the merits without having to be panicked into a political decision."

The war has taken an enormous toll on Bush's own standing. Most Americans polled think the U.S. invasion of Iraq was a mistake.

Security has improved there, credited largely to the increase in U.S. military might last year. But the pace of political progress has exasperated U.S. lawmakers and military leaders.

The U.S. has about 158,000 troops in Iraq. That number is expected to drop to 140,000 by summer.

Bush met for two hours Monday with his national security team. Petraeus and Ryan Crocker, the U.S. ambassador to Iraq, took part by video linkup.

Meanwhile, both Democratic presidential contenders made note of the 4,000 deaths.

Sen. Hillary Rodham Clinton told a campaign audience in Pennsylvania that she would honor the fallen by ending the war and bringing home U.S. troops "as quickly and responsibly as possible." Her rival for the nomination, Sen. Barack Obama, said, "It is past time to end this war that should never have been waged by bringing our troops home, and finally pushing Iraq's leaders to take responsibility for their future."

Vice President Dick Cheney, meanwhile, criticized Clinton and Obama for advocating a U.S. withdrawal from Iraq. Without mentioning them by name, he said such a pullout would put the U.S. at risk.

"I do, and (it) is seriously, seriously misguided," Cheney said in an ABC News interview while traveling in the Middle East. "A belief that somehow we can walk away from Iraq, and it won't have lasting consequences -- I'm talking about any candidate for high office who believes the solution to our problem in that part of the world is to walk away from the commitments that we've made in Iraq, Afghanistan, and elsewhere. "

The Associated Press count of 4,000 deaths is based on U.S. military reports and includes eight civilians who worked for the Department of Defense.

Last year, U.S. military deaths spiked as troops sought to regain control of Baghdad and surrounding areas. The death toll has seesawed since, with 2007 ending as the deadliest year for American troops at 901 deaths. That was 51 more than 2004.

Tens of thousands of Iraqi civilians have been killed.

Putting the 4,000 U.S. deaths in context, 406,000 U.S. service member died in World War II, 58,000 were killed in Vietnam, and 37,000 died in Korea.

The White House sought to ensure that Bush thinks of each death individually, and that "every single loss is tragic," as spokeswoman Dana Perino put it.

"You regret every casualty, every loss," Cheney said. "The president is the one that has to make that decision to send young men and women into harm's way. It never gets any easier."