Baghdad - Two car bombs exploded five minutes apart in a busy Baghdad market in a mainly Shiite district Sunday, killing at least 18 people and wounding 50, while north of the capital, two US helicopters crashed after an apparent collision in the air. Two soldiers were killed and five wounded, the military said.

A woman walks near the scene of a car bomb attack in Kerbala, 110 km (70 miles) south of Baghdad April 14, 2007. [Reuters]

The bombing in the southwest Baghdad market opened Sunday's renewal of sectarian carnage a day after a bomb killed at least 37 people and wounded more than 150 near a bus station and market close to one of Shiite Islam's holiest shrines in Karbala.

The two American helicopters crashed after an apparent collision 12 miles north of Baghdad.

"An investigation will be conducted to determine the cause of the incident; however, initial reports indicated it appears to be from a mid-air collision and not the result of enemy fire," the military said in a statement.

Also Sunday, a suicide bomber blew himself up in a small bus in northwest Baghdad and killed at least eight people and wounded 11, police and hospital officials said.

A suicide bomber also struck in Baghdad on Saturday, blowing up his car on a major bridge and killing 10 in the second such attack in 48 hours.

Chaotic arguing erupted in Iraq's legislature during its Saturday session, with the parliament speaker shouting for order as lawmakers squabbled over who was to blame for holes in security that allowed a suicide bomber to mingle among them Thursday and kill a Sunni Arab lawmaker.

The political wrangling underlined the continuing weakness of Prime Minister Nouri al-Maliki's government despite a more than 2-month-old US-Iraqi military operation intended to pacify Baghdad and give his regime room to function.

The crackdown, which will land 30,000 additional American troops in Iraq by the end of next month, comes as opposition to the strategy grows in Washington and emerges as a central issue in the US presidential campaign.

A possible presidential contender and one of the most vocal Republican critics of President Bush's Iraq policy, Sen. Chuck Hagel of Nebraska, was in Baghdad and planned to hold a news conference here Sunday. It was his fifth trip to the war zone.

In an interview broadcast Saturday, al-Maliki said he believed US support for his administration was steadfast.

"I feel that there is strong support because success would mean a civilized and democratic process," he told Al-Arabiya television. "I don't feel any change ... despite differences within the American government."

The crackdown also brought a Pentagon decision this past week to extend the deployments of US troops from 12 to 15 months -- a situation that the US commander in Iraq acknowledged Saturday was "tough news."

In a letter to his troops, Gen. David Petraeus expressed appreciation for "the hardship and strain the extension will put on you and your families," and he warned of "an enormous amount of hard work ahead."

In addition to the bombings in Karbala and Baghdad, at least 40 people were killed or found dead across Iraq on Saturday. The US military announced the death of one service member, killed Friday by a roadside bomb in southern Iraq.

Saturday's bloodshed in Karbala came when a parked car loaded with explosives blew up at a busy bus station at midmorning, killing at least 37 people and wounding 168, police and hospital officials said. Other reports put the death toll as high as 56.

The station is about 200 yards from one of Shiite Muslims' holiest spots -- the Imam Hussein shrine, where the Prophet Muhammad's grandson is buried. The shrine, 50 miles south of Baghdad, is the destination of an annual Shiite pilgrimage, during which hundreds of faithful were slain last month.

A makeshift triage center was set up in tents near the blast site. A man guided a wooden cart piled with body parts through a tangle of IV bags. The charred body of a child lay motionless on a stretcher.

At least 16 children were among the dead, said Interior Ministry spokesman, Brig. Gen. Abdul-Karim Khalaf. Iranian and Pakistani pilgrims were also among the casualties, hospital officials said.

"I want my father. Where is my father?" cried Sajad Kadhim, an 11-year-old lying on the ground as doctors treated his burns.

"All I remember was we were shopping. My father was holding my hand and suddenly there was a big explosion. I don't know where my father is. I want my father," the boy cried.

Mourners swarmed ambulances, beating their chests and crying out in grief. Some stormed the Karbala governor's office, demanding his resignation and blaming local authorities for lax security. Two police vehicles were set afire.

A spokesman for a top Shiite cleric in Karbala, Mohammed Taqi al-Mudarsi, said three civilians were killed in clashes with police.

"The behavior of Iraqi security forces was uncivil," said the spokesman, Ahmed Al-Shakarji. "People were trying to rescue their relatives and friends ... but the security forces opened fire on them."

The suicide car bombing in Baghdad killed 10 people, police said. The concrete structure of the Jadriyah bridge sustained little damage.

On Thursday, a suicide truck bomb collapsed the steel-girder al-Sarafiyah bridge farther north along the Tigris River, plunging cars into the water and killing 11 people.

Parliament convened a regular session a day after an emergency meeting was held to express defiance to insurgents and mourn those wounded when a suicide bomber blew himself up in the assembly's cafeteria Thursday.

But Saturday's session descended quickly into chaos and angry recriminations. Speaker Mahmoud al-Mashhadani said his office took "full responsibility" for Thursday's security breach, but he reminded legislators that some of them have refused to be searched when entering the building.

Khalaf, the Interior Ministry spokesman, told reporters that the bomber was known to legislators and that they identified his body after the explosion. He did not elaborate. Media speculation has fallen on workers in the building or a lawmaker's bodyguard.

A group with links to al-Qaida posted a Web statement Saturday claiming to have kidnapped 20 Iraqi soldiers to avenge the alleged rape of a woman by police. The Islamic State of Iraq threatened to kill the hostages unless the government handed over the alleged rapists within 48 hours.

The statement was accompanied by color photographs showing 20 blindfolded men in uniform. The claim and photos could not be independently verified, and there were no reports of kidnappings Saturday.