BAGHDAD - A suicide car bomber struck in the main Shiite district of the capital yesterday, killing at least nine people as the U.S. military said its troop buildup in Baghdad was nearly complete. Three more U.S. soldiers were killed by bombs in the capital.

At least 85 Iraqis were killed or found dead nationwide, police reported. The dead included eight people killed when a roadside bomb destroyed their minibus about 20 miles 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 cloud of black smoke 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-Qaeda in Iraq. Such extremists consider Shiites heretics and collaborators with the Americans.

Also yesterday, two U.S. soldiers were killed and two were wounded when a bomb devastated their vehicle in southern Baghdad, the U.S. command said. A third soldier died in a blast in western Baghdad, the command said.

At least 3,354 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count. Last month, at least 104 U.S. service members died - the highest monthly figure since December.

Persistent bombings threaten to undermine the 11-week U.S. effort to restore order in the capital, which was struck by a wave of Sunni-Shiite slaughter last year.

"The explosions show the incompetence of the security plan," said Saif Abdul-Khaliq, 28, who owns a stationery shop near the Sadr City blast site. "We expected security from this plan, but the only thing we got was traffic jams and blasts."

U.S. officials 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 violence, U.S. and Iraqi officials presented an upbeat picture of the security situation in the capital, saying sectarian killings were down while acknowledging the threat from the bombers.

"We continue to see a reduced total number of sectarian incidents in comparison to before the Baghdad security operation, including murders and kidnappings," said Rear Adm. Mark Fox said, citing the opening of 57 joint security stations and combat outposts to protect civilians in the Baghdad area.

Brig. Gen. Qassim al-Moussawi, an Iraqi military spokesman, said attacks had fallen significantly in Baghdad as extremists flee the city.

He said that most of the fighting was occurring in communities near the capital and that Iraqi forces would bolster their positions within the city shortly.

U.S. officials have insisted it is too early to judge the effectiveness of the security plan because all American forces will not be deployed in the streets until next month.

Yesterday, the U.S. 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 Bush in January.

About 3,700 soldiers from the Fourth Brigade of the Second Infantry Division, based in Fort Lewis, Wash., will be deployed in the Baghdad area and in northern Iraq, the military said.

When the fifth brigade arrives by next month, the U.S. command will have about 160,000 troops in the country.