U.S. President George W. Bush said the 4,000 Americans killed in Iraq "laid the foundation for peace for generations to come" by sacrificing their lives.

His comment came a day after the U.S. death toll in Iraq reached the grim milestone of 4,000 dead when four soldiers were killed by a roadside bomb in Baghdad, days after the war entered its sixth year.

He made his comment to the U.S. State Department following a two-hour briefing on Iraq-U.S. diplomatic efforts around the world.

"I hope their families know that citizens pray for their comfort," Bush said.

He added that every life is precious, and he would work hard to make sure "there is an outcome that will merit the sacrifice."

U.S. Vice-President Dick Cheney acknowledged the effects of the death toll on the American public.

"You regret every casualty, every loss," he 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."

The four soldiers were on patrol when their vehicle struck the bomb around 10 p.m. Sunday in the southern part of the city. Another soldier was injured in the attack.

Navy Lt. Patrick Evans, a military spokesman, expressed condolences to all the families of soldiers killed in Iraq, saying each death is "equally tragic."

"There have been some significant gains. However, this enemy is resilient and will not give up, nor will we," he said. "There's still a lot of work to be done."

Deaths of U.S. soldiers jumped last year as the army tried to regain control of Baghdad and the surrounding area, generating the deadliest annual toll, with 901 Americans killed.

The second-worst year since the U.S.-led invasion began on March 20, 2003, was in 2004, with 850 deaths.

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 Defence.

Also on Sunday, rocks and mortars pounded the Green Zone — a U.S.-protected part of Baghdad that houses the U.S. Embassy and Iraqi government headquarters — and a suicide car bomber struck an Iraqi army post in the northern city of Mosul.

The attacks were two among many nationwide on Sunday that killed at least 78.

On Monday, the U.S. military blamed Iranian-backed Shia militia factions for the attacks, adding that the rockets fired into the Green Zone were Iranian-made and supplied by the Quds Force, an elite unit of Iran's Revolutionary Guards.

The U.S. military has accused Iran of arming and funding Shia extremists to fight American forces in Iraq. Iran denies the allegation.

The number of Iraqi civilians killed since the U.S.-led invasion has been difficult to determine due to a lack of accurate information.

One tally by Iraq Body Count, a lobby group that monitors civilian deaths through media reports and official figures, estimates that 82,349 to 89,867 Iraqi civilians have been killed in the conflict.

With files from the Associated Press