BAGHDAD &

U.S. officials said today they will press forward in the fight against extremists in Iraq a day after the overall U.S. death toll in the five-year conflict rose to 4,000.




The White House called the grim milestone "a sober moment" and said President Bush spends time every day thinking about those who have lost their lives in battle.




"He bears the responsibility for the decisions that he made," White House press secretary Dana Perino said. "He also bears the responsibility to continue to focus on succeeding."




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




No group claimed responsibility for the Green Zone attacks, but suspicion fell on Shiite extremists based on the location of the launching sites.




The deaths of four U.S. soldiers in a roadside bombing about 10 p.m. Sunday in southern Baghdad pushed to 4,000 the number of American service members killed as the war enters its sixth year. Another soldier was wounded in the attack, the military said.




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.




"You regret every casualty, every loss," Vice President Dick Cheney 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."




An American military official in Baghdad said each U.S. death is "equally tragic" and underscored the need to keep up the fight.




"There have been some significant gains. However, this enemy is resilient and will not give up, nor will we," military spokesman Navy Lt. Patrick Evans 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.




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 because of 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, highlighting the fragile security situation and the resilience of both Sunni and Shiite extremist groups.




The U.S. Embassy said two government employees &

an American and a Jordanian &

were seriously injured and six other people required medical attention after Sunday's volley of rocket attacks.




Local hospital and police officials said at least 12 Iraqis were killed and 30 more were wounded in rocket or mortar blasts that apparently fell short after being aimed at the Green Zone from scattered areas of eastern Baghdad.




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 the militia 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.




Also today, Iraqi Prime Minister Nouri al-Maliki relieved the top two security officials in Basra, Iraq's second largest city, officials said. The move is a sign of growing concern about security in the nation's oil capital since British forces handed over control of the city last year.




Two Iraqi officials said the police chief of Basra, Maj. Gen. Abdul-Jalil Khalaf, and the commander of the city's joint military-police operation, Lt. Gen. Mohan al-Fireji, have been replaced.




But the two officials spoke on condition of anonymity because they were not supposed to release the information.




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.




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Associated Press writer Bushra Juhi contributed to this report.