A suicide car bomber struck in the main Shiite district of the capital Wednesday, killing at least nine people as the US military said its troop buildup in Baghdad was nearly complete. Three more US soldiers were killed by bombs in the capital. At least 76 other people were killed or found dead nationwide, police reported. They included eight people killed when a roadside bomb destroyed their minibus about 30 kilometers south of Baghdad. The suicide attack occurred at dusk near a police station in Sadr City, the Baghdad stronghold of the Mahdi Army militia. Three policemen and six civilians were killed and 34 people were wounded, police said. Ahmed Mohammed Ali, 31, who sells ice cream and cigarettes in Sadr City, said the blast sent a chocking cloud of black smoke billowing into the air. "I saw police and civilian cars on fire," Ali said, "There were several wounded people, including women and children, and most of the wounds were caused by burns. There were charred bodies near pools of blood." No group claimed responsibility, but suicide bombings are generally associated with Sunni religious extremists led by al-Qaida, who consider Shiites heretics and collaborators with the Americans. Also Wednesday, two US soldiers were killed and two others were wounded when a bomb devastated their vehicle in southern Baghdad, the US command said. Another soldier died in a blast in western Baghdad, the command said. The deaths raised to at least 3,354 members of the US military who have died since the Iraq war started in March 2003, according to an Associated Press count. Last month, at least 104 US service members died - the highest monthly figure since December. Persistent bombings threaten to undermine the 11-week US effort to restore order in the capital, which was wracked by a wave of Sunni-Shiite slaughter last year that plunged the country into civil conflict. "The explosions show the incompetence of the security plan," said Saif Abdul-Khaliq, 28, who owns a stationary shop near the Sadr City blast site. "We expected security from this plan, but the only thing we got was traffic jams and blasts." US officials also fear the bombings will provoke a violent response from Shiite militiamen, who have generally assumed a lower profile in the capital since the crackdown began Feb. 14. Despite the latest carnage, US and Iraqi officials presented an upbeat picture of the security situation in the capital, insisting that sectarian killings were down, while acknowledging the threat from the bombers. "While the security situation remains exceedingly challenging, we've seen some encouraging signs of progress," Rear Admiral Mark Fox said, citing the opening of 57 joint security stations and combat outposts to protect civilians in the Baghdad area. "We continue to see a reduced total number of sectarian incidents in comparison to before the Baghdad security operation, including murders and kidnappings," Fox said. But he acknowledged that car bomb attacks have increased, including some with very high casualties. Brig. Gen. Qassim al-Moussawi, an Iraqi military spokesman, said extremist attacks had fallen significantly in Baghdad as militants flee the city. He said most of the fighting was occurring in communities near the capital. "Next week will witness more military operations in both halves of Baghdad," he said, referring to the two sides of the Tigris River that divides the city. "Almost all our military operations are now taking place on Baghdad's outskirts." US officials have insisted that it's too early to judge the effectiveness of the security plan because all American forces will not be deployed in the streets until next month. On Wednesday, the US military announced that its buildup of forces was nearly complete with the arrival this week of the fourth of five brigades ordered to Baghdad by President George W. Bush in January. About 3,700 soldiers from the 4th Brigade of the 2nd Infantry Division, based in Fort Lewis, Washington, will be deployed in the Baghdad area and in northern Iraq, the military said. When the fifth brigade arrives by next month, the US command will have about 160,000 American troops in the country. Still, the security operation has not ended the slaughter. On Wednesday, police reported finding the bullet-riddled bodies of 55 people apparently slain by sectarian death squads. They included 30 in Baghdad and 10 in Baqouba, where US troops are trying to wrest control of the city from al-Qaida and its allies. The other killings were reported in Mosul, Baghdad and communities south of the capital, police said. The violence occurred on the eve of an international conference in the Egyptian resort of Sharm el-Sheik in a bid to boost economic and diplomatic support for Iraq. Secretary of State Condoleezza Rice acknowledged international resistance to new financial and political support for Iraq - particularly debt relief. "The region has everything at stake here; Iraq's neighbors have everything at stake here," Rice told reporters traveling with her to a gathering that will include US adversaries Iran and Syria.