BAGHDAD - Three parked cars exploded within 30 minutes in a predominantly Shiite area of Baghdad on Monday, killing at least 12 people, police said, the deadliest in a series of bombings and shooting attacks nationwide.

On Baghdad's northeastern outskirts, meanwhile, residents of a Shiite militia stronghold complained that their neighborhood was under siege by U.S. and Iraqi forces. The U.S. military said vehicle traffic was not being permitted in the Husseiniyah area but it was now letting residents walk or use donkey carts to collect food.

The U.S. military also reported the deaths of four American troops this weekend. Three soldiers were killed Saturday in roadside bombings in Baghdad, south of the capital and the northern city of Samarra. A Marine was killed in combat Saturday in Anbar province.

At least 42 Iraqis were killed nationwide on Monday, according to security officials who asked not to be identified because they feared retribution.

Two of the blasts in the Baghdad neighborhood of Karradah struck nearly simultaneously.

One targeted a passing police patrol, killing three officers and three pedestrians and wounding nine other people, a police officer said, speaking on condition of anonymity because he was not authorized to disclose information. The officer said at least seven cars were damaged in the blast, which struck near to the Interior Ministry's nationality and social affairs directorate and the 14th of July bridge.

Another parked car bomb about 500 yards away struck at about the same time, ripping through a bustling market of vegetables and household goods, killing three civilians and wounding five others, the policeman said.

AP Television News footage showed U.S. soldiers milling about the charred wreckage, with shattered glass and blackened debris from nearby shops and street stalls strewn on the bloodstained pavement.

Another car packed with explosives struck a police patrol in Elwiyah square at about 11:30 a.m. in another part of Karradah, killing two policemen and a civilian and wounding five people, police said.

Karradah, a popular shopping area, has been hit by several high-profile bombings, and Monday's attack occurred despite a 5-month-old U.S.-Iraqi security operation aimed at stopping such violence in the capital.

Hassan Sami, a 28-year-old clothing store owner in Karradah, said he was showered by shattered glass and wounded in the left arm.

"Nothing was left except the smell of charred flesh mixed with gun powder and wreckage stained with blood," Sami said. "We've been attacked many times before and the government can't do anything for this area, it only sends its patrols who roam the streets with their annoying sirens without doing anything useful."

Another car packed with explosives blew up on the main road about 200 yards from an entry point to the U.S.-controlled Green Zone, killing at least four Iraqis and wounding seven, a police officer said, speaking on condition of anonymity because of security concerns. The heavily fortified Green Zone is home to the U.S. and British embassies as well as Iraqi government offices and thousands of American troops and contractors.

Elsewhere in the capital, a bomb exploded on a minibus near a busy commercial area, killing one person and wounding nine others, police said.

A roadside bomb struck an Iraqi army patrol about 75 miles east of Baghdad, near the Iranian border, killing five troops, according to police and morgue officials. The explosion occurred on the southeastern edge of the volatile Diyala province.

Also near the Iranian border, gunmen ambushed a convoy of trucks loaded with goods being sent from major wholesale markets in Baghdad to Khanaqin, 90 miles northeast of Baghdad. Five people were killed and three others kidnapped, including drivers and guards, police said.

In western Anbar province, security officials said at least two policemen were killed and 10 wounded when a woman hiding an explosives belt under her Islamic gown blew herself up as she was about to be searched at a checkpoint. Although suicide bombings regularly claim scores of victims in Iraq's sectarian violence, female bombers remain relatively rare.

Iran's ambassador to Baghdad, meanwhile, confirmed that the United States and Iran will discuss the security situation in Iraq on Tuesday in Baghdad, the official Islamic Republic News Agency reported.

"The composition of the negotiating teams will include ambassadors of Iran and America in Baghdad, as head of the two teams, with observance of Iraqi officials," IRNA quoted Hasan Kazemi Qomi as saying Monday.

Iraqi politicians and radical Shiite cleric Muqtada al-Sadr's office called on the government and humanitarian organization to stop what they called a siege on Husseiniyah, and residents complained of shortages and said they had been warned by authorities to flee the area.

Um Bassim, a 51-year-old housewife, said her family had decided to stay but moved into two rooms at the back of their house to stay safe and was relying on reserves of food and fuel, including gas siphoned from their idle car for use in the generator.

"Because there is no electricity, we use the generator two hours a day to save the fuel for the coming days because we do not know how long this situation will last," she said in a telephone interview.

She said no military operations had occurred yet on the ground, but a curfew had been imposed since Friday and they could hear aircraft flying overhead.

"My husband offered to take us out and return to protect our house and belongings but we refused to leave because we would be so worried about him," she added.

Lt. Col. John Drago, the U.S. commander in the area, said militants had used the area to attack American forces but denied a siege was under way. He said traffic was restricted to emergency vehicles but commercial vendors were being allowed to bring food to the south of Husseiniyah.

"Civilians are authorized to walk to these vendors to buy food. Donkey carts may be used, but no vehicle movement is authorized," he said in an e-mail. "We are also allowing civilians that need medical aid, to walk to the Hamid Shaub Hospital for free treatment."