WASHINGTON (CNN) -- President Bush expressed sympathy Monday for the families of the 4,000 Americans killed in the war in Iraq, promising to make sure their loved ones "were not lost in vain."

President Bush addresses the press Monday with Secretary of State Condoleezza Rice.

"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 told reporters after a meeting at the State Department.

Military officials reported four U.S. soldiers died Sunday in a roadside bombing in Iraq, bringing the American toll in the war to the milestone of 4,000 deaths, including eight Defense Department civilians.

The war entered its six year last week. Watch more of Bush's remarks »

Of the 4,000 U.S. military personnel killed in the war, 3,263 have been killed in attacks and fighting and 737 in nonhostile incidents, such as traffic accidents and suicides.

The numbers are based on Pentagon data counted by CNN.

During a briefing with reporters Monday, White House press secretary Dana Perino said the president "definitely feels the loss."

"He gets a report about every single soldier who passes away," she said. "And he always pauses a moment to think about them and to offer a prayer for their loved ones and their family and friends."

Bush also held a two-hour National Security Council videoconference Monday with his top commander in Iraq, Gen. David Petraeus, and Ryan Crocker, the U.S. ambassador in Baghdad.

Perino said the meeting focused on upcoming congressional testimony from Petraeus and Crocker, scheduled for April 8 and 9.

In Baghdad, a U.S. military spokesman said every death is "keenly felt" by American commanders, families and friends.

"No casualty is more or less significant than another; each soldier, Marine, airman and sailor is equally precious and their loss equally tragic," Rear Adm. Gregory Smith said.

In a statement issued Monday, House Speaker Nancy Pelosi said the continued cost of the war in blood and treasure has become "unacceptable."

"With 4,000 American lives lost and thousands injured, many of them permanently, 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 Pelosi, D-California.

Earlier in the day, the Democratic presidential candidates commented on the death toll reaching 4,000.

"I want to take a moment to note yesterday's heartbreaking news that five years after the start of the war, there have been 4,000 U.S. military deaths in Iraq," Sen. Hillary Clinton said at the start of a speech on the economy in Philadelphia, Pennsylvania, on Monday.

"Tens of thousands of our brave men and women have also suffered serious wounds, both visible and invisible, to their bodies, their minds and their hearts. As president, I intend to honor their extraordinary service and the sacrifice of them and their families by ending this war and bringing them home as quickly and responsibly as possible," she said. Watch Sens. Clinton and John McCain comment on Iraq »

Her rival for the Democratic nomination, Sen. Barack Obama, also commented on the the "grim" milestone, saying "each death is a tragedy, and we honor every fallen American and send our thoughts and prayers to their families."

"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," he said.

Republican candidate Sen. John McCain recently returned from the Mideast.

"The central battleground in the battle against al Qaeda is in Iraq today," McCain said, noting a message allegedly released by Osama bin Laden last week calling Iraq 'a perfect base.'

"My Democrat opponents who want to pull out of Iraq refuse to understand what's being said and what's happening, and that is, the central battleground is Iraq in this struggle against radical Islamic extremism," McCain said. E-mail to a friend

CNN White House Correspondent Elaine Quijano contributed to this report.

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