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BAGHDAD -- Four U.S. soldiers were killed when a bomb hit their vehicle in south Baghdad late Sunday, bringing the number of U.S. service members killed in the Iraq war to 4,000.

The grim milestone came at a time when attacks against the U.S. military are ebbing and officials have claimed significant progress against Iraq's deadly insurgency and sectarian violence. It was reached about 10 p.m. on a day when more than 60 Iraqis were killed and dozens injured in attacks in Baghdad and north of the capital.

The U.S.-protected Green Zone in Baghdadcame under heavy mortar or rocket fire. There were no immediate reports of casualties inside the fortified enclave, which houses the U.S. Embassy, government offices and military bases. But Iraqi police said a number of the projectiles missed their apparent target and caused casualties in neighboring districts.

At least 426 of the Americans killed in the war were from California, more than any other state, and 98 of them were women, according to the independent website icasualties.org. Those figures do not include the soldiers killed Sunday, whose identities were withheld pendingnotification of relatives.

Last year was the deadliest of the 5-year-old war for U.S. troops, with 901 killed, according to the website. U.S. commanders had warned that the number of casualties would increase after President Bush ordered an additional 28,500 U.S. troops deployed in an effort to quell the civil war.

Many of the extra troops were assigned to small neighborhood outposts, to build better ties and provide a continuous presence in the areas they protect. The strategy left the troops more vulnerable to attackers, such as a suicide bomber who killed five soldiers as they chatted with Baghdad shopkeepers March 10.

But commanders say the strategy has yielded better intelligence about insurgent activities. The number of attacks in Iraq has dropped about 60% since the number of U.S. troops reached nearly 170,000 in June, according to the military. But American officials have warned that the progress is reversible and militants will continue to wage attacks as most of the additional troops leave by the end of July.

"There is no question that there has been significant progress in the security arena, but we see daily examples of the fact that Al Qaeda and other insurgent groups and militia criminals also remain very, very dangerous," Gen. David H. Petraeus, top U.S. commander in Iraq, said this month.

At least 27 U.S. personnel have been killed so far in March, down from more than 100 a month in April, May and June, according to icasualties.org. Dozens more have been wounded, including one in Sunday night's attack.

Homemade bombs like the one used Sunday remain the biggest killers of U.S. forces, accounting for at least 1,733 deaths, according to icasualties.org.

The number of Iraqi deaths far outpaces the American toll. At least 8,037 Iraqi soldiers and policemen have been killed, according to the U.S.-based Brookings Institution. Estimates of the civilian death toll have ranged from tens of thousands to more than half a million.

In Sunday's worst attack, a suicide bomber blew up a tanker laden with explosives at the entrance to an Iraqi army base in Mosul, a northern city described by the U.S. military as the last urban stronghold of Sunni militants loyal to the group Al Qaeda in Iraq.

At least 12 Iraqi soldiers were killed and 30 others injured along with 12 civilians, said army Brig. Gen. Mohammed Ahmed.

"The bombing was very powerful," taxi driver Khalid Azzawi said from a hospital bed. "I lost control and crashed into the sidewalk."

Another suicide car bomber attacked an Iraqi army checkpoint in Mosul, killing one officer and injuring 10 other people, police said.

Militants pounded the capital with at least 16 rockets and seven mortar rounds Sunday, including three barrages aimed atthe Green Zone, the Interior Ministry said. U.S. officials did not immediately say who was responsible. The strikes killed at least 13 people and injured 29, police said.

Gunmen in three cars sprayed bullets at commuters waiting to board minibus taxis, killing seven people and injuring 16 others in the mostly Shiite southeastern neighborhood of Zafaraniya, police said.

In the northwestern Shiite neighborhood of Shula, police said a suicide car bomber attacked a line of people waiting for gasoline, killing seven of them and injuring 12. The U.S. military described the method of attack as a parked car bomb and put the toll at five dead and seven wounded.

U.S.-led forces traded fire with suspected insurgents during an operation east of Baqubah targeting members of a suicide bombing network, killing 12 people, the military said in a statement. Iraqi police and hospital officials put the toll at 15 and said the victims included women and children.

alexandra.zavis@latimes.com

Special correspondent Ruaa al-Zarary in Mosul and others in Baghdad and Kirkuk contributed to this report.