BAGHDAD — A roadside bomb killed four U.S. soldiers in Baghdad on Sunday, the military said, pushing the overall American death toll in the five-year war to at least 4,000.

The grim milestone came on the same day that rockets and mortars pounded the U.S.-protected Green Zone, underscoring the fragile security situation and the resilience of both Sunni and Shiite extremist groups despite an overall lull in violence.

A Multi-National Division — Baghdad soldier also was wounded in the roadside bombing, which struck the soldiers’ patrol vehicle about 10 p.m. in southern Baghdad, according to a statement.

Identities of those killed were withheld pending notification of relatives.

Navy Lt. Patrick Evans, a military spokesman, expressed condolences to all the families who have lost a loved one 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.”

The 4,000 figure is according to an Associated Press count that includes eight civilians who worked for the Department of Defense.

Last year, the U.S. military deaths spiked along with the Pentagon’s “surge” — the arrival of more than 30,000 extra troops trying to regain control of Baghdad and surrounding areas. The mission was generally considered a success, but the cost was evident as soldiers pushed into Sunni insurgent strongholds and challenged Shiite militias.

Military deaths rose above 100 for three consecutive months for the first time during the war: April 2007, 104; May, 126 and June at 101.

The death toll has seesawed since, with 2007 ending as the deadliest year for American troops at 901 deaths. That was 51 more deaths than 2004, the second deadliest year for U.S. soldiers.

The milestones for each 1,000 deaths — while an arbitrary marker — serve to rivet attention on the war and have come during a range of pivotal moments.

When the 1,000th American died in September 2004, the insurgency was gaining steam. The 2,000-death mark came in October 2005 as Iraq voted on a new constitution. The Pentagon announced its 3,000th loss on the last day of 2006 — a day after Saddam Hussein was hanged and closing a year marked by rampant sectarian violence.