BAGHDAD — The overall U.S. death toll in Iraq rose to 4,000 after four soldiers were killed in a roadside bombing in Baghdad, a grim milestone that is likely to fuel calls for the withdrawal of American forces as the war enters its sixth year.

The American deaths occurred Sunday, the same day rockets and mortars pounded the U.S.-protected Green Zone in Baghdad and a wave of attacks left at least 61 Iraqis dead nationwide.

An Iraqi military spokesman said Monday that troops had found rocket launching pads in different areas in predominantly Shiite eastern Baghdad that had been used by extremists to fire on the Green Zone, which houses the U.S. Embassy and the Iraqi government headquarters.

"We hope to deal with this issue professionally to avoid civilian casualties," said spokesman Qassim al-Moussawi.

The four soldiers with Multi-National Division — Baghdad were on a patrol when their vehicle was struck at about 10 p.m. Sunday in southern Baghdad, the U.S. military said. Another soldier was wounded in the attack — less than a week after the fifth anniversary of the conflict.

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."

Last year, U.S. military deaths spiked as U.S. 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 deaths than 2004, the second deadliest year for U.S. soldiers.

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.

Tens of thousands of Iraqi civilians also have been killed since the U.S.-led invasion on March 20, 2003, although estimates of a specific figure vary widely due to the difficulty in collecting accurate information.

One widely respected tally by Iraq Body Count, which collects figures based mostly on media reports, estimates that 82,349 to 89,867 Iraqi civilians have lost their lives in the conflict.

Overall attacks also have decreased against Iraqi civilians but recent weeks have seen several high-profile bombings, underscoring the fragile security situation and the resilience of both Sunni and Shiite extremist groups.

Mosul, Iraq's third largest city about 225 miles northwest of Baghdad, has been described as the last major urban area where the Sunni extremist al-Qaida group maintains a significant presence.

The persistent violence has led to strong public opposition to the war in the United States, with both Democratic presidential hopefuls Sens. Barack Obama and Hillary Clinton promising a quick pullout if they are elected.

President Bush has insisted the decline in violence shows his strategy is working and needs more time, a position taken by Republican presidential candidate Sen. John McCain.

Iraq's National Security Adviser Mouwaffak al-Rubaie said he sympathized with the American losses but warned against pulling out U.S. troops before Iraqi forces are ready to take over their own security and the situation is sufficiently stable.

"Honestly, this war is well worth fighting. This war, we are talking about war against global terror," he said Sunday in an interview with CNN.

No group claimed responsibility for the Green Zone attacks, but suspicion fell on Shiite extremists based on the eastern areas from which the weapons appeared to have been fired.

At least 10 civilians were killed and 20 more were wounded in rocket or mortar blasts in scattered areas of eastern Baghdad, some probably due to rounds aimed at the Green Zone that fell short.

The U.S. Embassy said at least five people were injured but no Americans were reported killed in the Green Zone attacks, which sent dark plumes of smoke rising over the district in the heart of the capital.

A U.S. official, speaking on condition of anonymity because the official was not authorized to release the information, said those injured included an American and four third-country nationals, meaning they were not American, British or Iraqi.

The heavily fortified area has frequently come under fire by Shiite and Sunni extremists, but the attacks have tapered off as violence declined over the past year.

The attacks followed a series of clashes last week between U.S. and Iraqi forces and factions of the Mahdi Army, the biggest Shiite militia loyal to radical cleric Muqtada al-Sadr.

Al-Sadr has declared a cease-fire through mid-August to purge the militia of criminal and dissident elements but it has come under severe strains in recent weeks.

Al-Sadr's followers have accused the Shiite-dominated government of exploiting the cease-fire to target the cleric's supporters in advance of provincial elections expected this fall and demanded the release of supporters rounded up in recent weeks.