BAGHDAD, Iraq — The Iraqi government formally launched a long-awaited security crackdown in Baghdad yesterday, with US and Iraqi troops stepping up patrols, establishing new checkpoints and randomly searching cars to stop the violence in the capital.Prime Minister Nouri al-Maliki said the sweep, code-named Operation Imposing Law, would target those "who want to continue with rebellion."

There were conflicting reports, meanwhile, about the whereabouts of Muqtada al-Sadr, whose militia have been blamed for some of the worst sectarian killings in the past year, after a US official said the radical Shiite cleric had fled to Iran ahead of the security operation.

Maj. Gen. William Caldwell, the chief US military spokesman, insisted that al-Sadr had left the country, although he declined to comment on the reasons or give other details.

"We will acknowledge that he is not in the country and all indications are in fact that he is in Iran," Caldwell told reporters in Baghdad.

But several of the cleric's supporters denied the reports, with one official saying the cleric had met with government officials late Tuesday in the Shiite holy city of Najaf, where he has his headquarters.

Lawmaker Nassar al-Rubaie, the head of Sadrist bloc in parliament, also insisted al-Sadr had not left the country.

"The news is not accurate because Muqtada al-Sadr is still in Iraq and he did not visit any country," lawmaker Nassar al-Rubaie, the head of Sadrist bloc in parliament, told The Associated Press.

Also yesterday, CNN reported that a Shiite militant group has issued a video of Iraqi-born US Army translator Ahmed Qusai al-Taayie, a 41-year-old reserve soldier from Ann Arbor, Michigan, who was abducted by gunmen on October 23.

The video was broadcast by CNN and it was unclear when it was made. Al-Taayie's uncle identified him from the video, the network said. The video did not immediately turn up in an AP search of militant websites.

Caldwell said officials were aware of the video and were analysing a copy of it.

A US soldier died yesterday after coming under small arms fire a day earlier from insurgents while on patrol north of Baghdad, the military said. Separately, another soldier died Tuesday in a non-combat-related incident that is under investigation, it said.

The deaths raise to at least 3,128 members of the US military who have died since the war began in March 2003, according to an AP count.

The US military announced last week that the clampdown had already begun, but Iraqis had seen little evidence of that before yesterday. President Bush has committed 21,500 more Americans to the operation, which is expected to involve a total of 90,000 Iraqi and US soldiers.

As the new checkpoints were set up in the city of six million of people, huge traffic jams developed and cars were forced to zigzag through soldiers' positions and barricades.

Dozens of people left their buses to cross the central Sinak Bridge on foot rather than wait for the vehicles to move through the jams. At one checkpoint, Iraqi troops stopped a convoy of three white SUVS that are commonly used by Iraqi government officials and checked their identification.

Al-Maliki discussed the security plan during a meeting with officials in the Shiite holy city of Karbala, 50 miles south of Baghdad.

"We have started today the new security plan. And we warn everyone who runs against it: Now we are ready to impose law on all those who want to continue with rebellion," he said. "Baghdad operations started today under code name 'Imposing Law."'

"By God willing, the new plan will have fruitful outcomes, not because of the use of force but because of those who declare their love to Iraq and its people," he said before going to the entrance of the Imam Hussein mosque to address thousands of supporters massed in the streets.

The pep talk came a day after the Iraqi commander of the Baghdad security crackdown, Lt. Gen. Abboud Qanbar, announced that Iraq will close its borders with Syria and Iran and ordered the return of unlawfully seized homes as part of the drive to end the violence that has threatened to divide the capital along sectarian lines.

Despite the stepped-up security, a parked car bomb struck a predominantly Shiite district elsewhere in central Baghdad, killing four civilians and wounding 10, police said.

In the western city of Ramadi, a suicide car bomber struck a police station, killing at least eight policemen and wounding seven, police said. The Warar station had just been rebuilt three months ago as part of reconstruction efforts in the volatile capital of Anbar province, an insurgent stronghold west of Baghdad.

A suicide car bomber also targeted an Iraqi army patrol in the northern city of Mosul, killing one soldier and four civilians and wounding 20 other people, police Brig. Abdul Karim al-Jubouri said.

Qanbar said Baghdad's nighttime curfew would be expanded by an hour, from 8 p.m. to 6 a.m., and permits allowing civilians to carry weapons in public would be suspended during all of the operation, which he suggested could last weeks.

The commander also said those who had occupied homes of displaced families would be given 15 days to return the properties to the original owner or prove they had permission to be there.

The general did not say when the borders would close, but another official said it was expected in two days. The official, who spoke on condition of anonymity because he was not authorised to speak to journalists, added that the borders would only partly reopen after the 72-hour closing.

The United States has long charged that Iran and Syria let extremists use their territory to slip into Iraq to attack US and Iraqi forces as well as civilians.

Iraqi authorities have routinely echoed the US charges against Syria, but they rarely make that claim about Iran, which has close ties with Iraq's Shiite-led government.