Bush mourns all 4,000 dead in Iraq - White House

President George W. Bush is saddened by the loss of 4,000 US troops in Iraq and will focus on ensuring the US succeeds in the 5-year-old conflict, the White House said.

"It's a sober moment, and one that all of us can focus on in terms of the number of 4,000," White House spokeswoman Dana Perino said after a roadside bomb in Iraq killed four US soldiers, pushing the US death toll to a new milestone.

"The president feels each and every one of the deaths very strongly and he grieves for their families," Perino said. "He obviously is grieved by the moment but he mourns the loss of every single life."

The 4,000th US death came days after Bush marked the fifth anniversary of the war and said the United States was on track toward victory.

The president chaired a meeting of his National Security Council and was to receive a briefing on the war from Ryan Crocker, the US ambassador in Baghdad, and Gen. David Petraeus, the top U.S. military officer in Iraq.

Bush and his advisors are trying to decide whether to continue to reduce the number of troops in Iraq after last year's surge, which is credited with lowering violence there.

Some experts believe a pause in troop reductions is necessary to avoid losing the gains made in recent months.

The deaths that pushed the US toll in Iraq to 4,000 happened as new violence burst out, including sustained mortar fire against the US protected "Green Zone" in Baghdad.

Perino said Bush feels responsible for leading the United States into the war and also for ensuring it succeeds.

"One of the things that he hears from families of the fallen is that they want him to lead the country to complete the mission and he is committed to doing that," Perino said. "He wants them to know that their sacrifices will not be in vain."