U.S. and Iraqi troops moved into a Sunni neighborhood in southern Baghdad on Thursday, while insurgents struck back with car bombs that killed seven people. In southern Iraq, British troops sealed off the border with Iran to prevent weapons smuggling.

Helicopters buzzed overhead as a joint U.S.-Iraqi force headed into the Dora neighborhood -- a longtime Sunni militant area -- on the second day of a long-awaited security operation in the capital, according to Iraqi officials. U.S. troops searched three Shiite areas Wednesday, meeting little resistance in house-to-house searches.

The Interior Ministry also said U.S. and Iraqi forces were sweeping through four main districts, including Sunni and Shiite areas, seizing weapons and ammunitions.

Defying the operation, two parked car bombs struck Dora near a major intersection with a highway leading to Shiite areas in the south, killing at least four civilians and wounding 15, police said. Oranges and other fruit from stalls at a nearby market were scattered among debris near the charred cars.

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 hurt.

Later, a car bomb exploded in the northeastern Baghdad district of Sadr City, killing three and injuring 17 others, police said.

In another attack, a car packed with explosives targeted an Iraqi patrol in the Sunni district of Jamiaa in western Baghdad, wounding two soldiers, while clashes erupted in another volatile Sunni area, leading to four arrests, police said.

In southern Iraq, security forces closed two border points with Iran at al-Sheeb and Shalamcha -- 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 Iraq's second-largest city about 340 miles southeast of Baghdad. The British military said the operation would last for 72 hours.

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President Bush said Wednesday the Iranian government is providing armor-piercing weapons to kill American soldiers in Iraq, although he backed away from claims the top echelon of Iran's government was responsible.

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

U.S. and Iraqi forces also detained several suspects in a raid targeting a Mahdi Army militia cell accused of sectarian kidnappings and killings, as well as roadside bomb attacks, in Mahru, near Suwayrah, 25 miles south of Baghdad, the military said.

Maj. Steven Lamb, deputy public affairs officer for the military command in Baghdad, said the 2nd Brigade Combat Team, 1st Cavalry Division was working with Iraqi national police doing cordon and search operations in Dora.

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

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

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

Mohammed Ali Jassim, a 40-year-old Sunni owner of a spare parts store, said he was hopeful it would work after he was forced to abandon his business 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 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 U.S. 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 a young girl about her taste in music after they found her doing homework on a couch, with a poster of Shakira on the wall.

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

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

"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. "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 that al-Sadr is in Iran, but denied he fled due to fear of arrest during the security crackdown. The assertion came a day after the U.S. military said he was believed to be in Iran, despite denials from his supporters.