BAGHDAD, Iraq - US and Iraqi forces expanded a security operation in Baghdad to a predominantly southern neighborhood in Baghdad on Thursday, but suspected Sunni insurgents struck back with car bombs that killed at least four people and sent smoke billowing over the skyline.

Iraqi army soldiers search a driver at a vehicle checkpoint in Baghdad, Iraq, Thursday, Feb. 15, 2007. [AP]

British troops, meanwhile, sealed off the southern city of Basra and closed two Iranian border crossings in a bid to choke off weapon supplies the Americans say are coming from Iran.

A joint US-Iraqi force headed into the southern Baghdad neighborhood of Dora - a longtime Sunni militant area - on the second day of the operation, according to Iraqi officials. US troops searched three Shiite areas on Wednesday, meeting little resistance as they searched house-to-house.

The Interior Ministry also said US and Iraqi forces were sweeping through four main districts of the capital, including Sunni and Shiite areas, at about 7 a.m., seizing various weapons and ammunitions.

Defying the security sweep, two parked car bombs struck Dora near a major intersection with a highway leading to Shiite areas in southern Iraq, killing at least four civilians and wounding 15, police said.

The blasts occurred about 80 yards from an Iraqi checkpoint on the southern edge of the district as patrols were passing, but no Iraqi forces were reported killed or wounded.

A car packed with explosives also targeted an Iraqi army patrol in the Sunni district of Jamiaa in western Baghdad, wounding two soldiers, while clashes erupted in another volatile Sunni area, leading to the detention of four suspects, police said.

In southern Iraq, security forces closed two border points with Iran at al-Sheeb and Shalamshen - blocking the gates with large metal containers - and expanded coastal patrols to monitor maritime traffic into southern Iraq, a statement said. Authorities also set up checkpoints around Basra and were targeting the most dangerous areas in the city, Iraq's second-largest 340 miles southeast of Baghdad.

The British military said the operation would last for 72 hours.

The commander of the Baghdad security crackdown, Lt. Gen. Abboud Qanbar, also has said Iraq will close its borders with Syria.

Iraqi army Brig. Qassim Moussawi, Qanbar's spokesman, said a joint US-Iraqi force had moved into the southern neighborhood of Dora. The US military said it could not comment on any ongoing operations.

Iraqi and security forces also intensified their presence elsewhere in eastern Baghdad and other areas of the capital, with vehicles and motorcycles subjected to comprehensive searches at checkpoints.

In western Baghdad, joint Iraqi and American patrols roamed the streets and stepped up searches but they showed no signs of sealing off neighborhoods in the areas, witnesses said.

Some Iraqis were optimistic about the plan, while others complained about the inconveniences imposed as traffic jams slowed movement in the capital.

Mohammed Ali Jassim, a 40-year-old Sunni owner of a spare parts store in Baghdad said he was hopeful it would work after he was forced to abandon his store in Sinak, one of Baghdad's commercial areas where more than 50 people were kidnapped by gunmen disguised in military uniforms late last year. Jassim's brother was among the victims.

"I wish I could open my store again and send my children to their schools without fear of being kidnapped or killed," said Jassim, who has three children and lives in Baghdad's western Khadra neighborhood. "The government must annihilate the Shiite militias which are supported by Iran and not turn eyes to their acts. The government should also seek political solutions not only military ones."

But Anwar Abdullah, 30, a supermarket owner in the predominantly Shiite eastern area of Mashtaal said the government had given the militants time to flee in advance of the operation, which was announced more than a month ago but only formally launched on Wednesday.

"Most of our clients have vanished since yesterday, just few of them showed up today. It sounds like we are going to be affected more than the terrorists by this security plan," Abdullah said. "The government has been talking about this plan for ages and many of the terrorists have fled Baghdad to other areas."

Thousands of US troops started a clearing operation in mostly Shiite areas north of the militia stronghold of Sadr City on Wednesday as new checkpoints went up across the city of 6 million people.

Some 2,500-3,000 troops - or an entire Stryker brigade - fanned out in the area, encountering little hostilities from the Iraqis who gathered on the street as they moved into houses, which ranged from squat structures with kitchens littered with dirty pots and pans to mansions with tall windows and spiral staircases.

Soldiers even teased one young girl about her music taste after they found her doing homework on a couch, wearing white and pink socks with a poster of Shakira on the wall.

Some people even left the doors open as the troops arrived, and little evidence of hostilities turned up other than some pictures of radical Shiite cleric Muqtada al-Sadr, an illegal bolt action rifle and a heavyset man watching an insurgent propaganda video that he said had appeared while he was channel surfing.

Al-Maliki, a Shiite, owes his job to al-Sadr and his failure to confront the sectarian violence carried out by the cleric's Mahdi Army militia had been partly blamed for the failure of two previous security operations. But the prime minister has promised not to let politics interfere with the current crackdown.

Senior Sunni politician Adnan al-Dulaimi expressed hope that the plan would succeed, but he accused the Shiite-dominated government of concentrating the crackdown on Sunni neighborhoods.

"We have noticed that this plan started with attacking Sunni neighborhoods in Baghdad in Ghazaliyah, Amiriyah, Jihad and Amil and conducting random arrests," al-Dulaimi said at a press conference. "It should concentrate on those inciting sedition, violence and terrorism in all areas without exception. And it should not only target Sunni neighborhoods."

An adviser to al-Maliki, Sami al-Askari, also said Thursday that al-Sadr is in Iran, but denied he fled due to fear of arrest during the escalating security crackdown. The assertion came a day after the US military claimed the radical Shiite cleric was believed to be in Iran, despite denials from the cleric's supporters.