BAGHDAD, Iraq — At least 18 civilians were killed yesterday in Baghdad when a truck filled with Shiite terrorist rockets accidentally blew up, American military officials said. It was the deadliest single explosion in the Iraqi capital in months.

Also yesterday, gunmen killed three American soldiers in the Sunni town of Hawijah, in northern Iraq, the military said, following a relative lull in troop deaths in May, when 19 Americans were killed.

The Baghdad incident occurred around 2 p.m. in the Shaab neighborhood just west of Sadr City, the stronghold of anti-American cleric Muqtada al-Sadr, whose terrorists have battled American troops in recent months.

Lieutenant Colonel Steve Stover, a spokesman for American forces in Baghdad, said Shiite terrorists were attempting to fire rockets at an American base from the back of a parked truck about 800 yards away. But he said the rockets, which were supposed to be detonated by a fuse or timer, exploded early.

The blast also wounded 29 people and destroyed 15 houses in a residential area around the truck, American officials said. Colonel Stover said most of the damage was caused by three rockets that exploded in the truck, though at least one rocket was launched. There were no American casualties.

“They were trying to hit us, but they really don’t care if something else happens and civilians are killed,” he said.

He said the attack was carried out by “special groups criminals,” a phrase American military often use to refer to rogue elements of Mr. Sadr’s Mahdi Army.

An aide to an Iraqi military spokesman, Brigadier General Qassim Atta, said initial reports suggested that the explosion was caused by a booby-trapped car and that eight people were killed and 50 wounded. The aide, who spoke on condition of anonymity, could not provide further details.

Also yesterday, a parked car packed with explosives blew up in front of a restaurant in the capital’s upscale Jadriya neighborhood, killing at least three police officers and engulfing several cars in flames, according to police.

A 44-year-old doctor, Um Ahmad, said she was walking toward a supermarket at about 8:15 p.m. when she heard an enormous boom. Her car was destroyed and her son and sister, who were with her, were hospitalized.

“The scene was horrible,” she said. “I saw people burning inside their cars.”

The evening had begun placidly, with families doing their food shopping and eating ice cream to beat the Baghdad heat.

But following the blast, enormous plumes of smoke swirled in the dark sky and the entire area was cordoned off. Seeking safety, people scurried away to their homes.