Thousands of U.S. troops swept house-to-house through mostly Shiite areas virtually unopposed Wednesday in the opening phase of the long-awaited Baghdad security crackdown.

Iraqi soldiers and police set up new checkpoints across the city of 6 million people, snarling traffic and forcing people to walk across bridges jammed with cars and trucks.

The U.S. military said 14 suspects were detained and four weapons caches discovered during the day's operation — seemingly a low tally. But U.S. officials say they are more concerned about establishing a long-term presence in the areas so that the public will gain confidence in security forces to protect them.

Outside the capital, fighting continued.

The military said four U.S. soldiers were killed Wednesday in an explosion in Diyala province, among six new U.S. deaths announced by the military. U.S. officers have expressed concern that insurgents and militias are leaving Baghdad to transfer the fight to Diyala and other provinces that border the capital.

Iraq's Sunni vice president, Tariq al-Hashemi, warned that advance publicity on the security operation had given Shiite militias time to flee the city for bases elsewhere in the country.

"I have information that numerous of their leaders are now in Basra and other southern provinces in safe havens," he told Al-Arabiya television. "I believe that those who were behind the bloodshed and the chaos should be pursued and criminals must face justice."

At least 38 Iraqis also were killed or found dead nationwide, including four civilians who died when a parked car bomb struck a predominantly Shiite district in central Baghdad. Only five bullet-riddled bodies were found on the streets of the capital, an unusually low number of apparent victims of so-called sectarian death squads mainly run by Shiite militias that have killed thousands in the past year.

The Baghdad neighborhoods targeted by the Americans — Shaab, Ur and Baida — lie north of the Shiite militia stronghold of Sadr City, which had been off-limits until Prime Minister Nouri al-Maliki lifted his protection of the notorious Mahdi Army, the largest Shiite militia.

Last year, U.S. soldiers came under intense sniper fire in those neighborhoods from Mahdi Army militiamen who were expanding into Shiite areas outside Sadr City.

This time, however, Iraqis watched in curiosity as some 2,500-3,000 troops — or an entire Stryker brigade — fanned out in the area, going house-to-house looking for weapons or suspected militia fighters as part of what it called "Operation Law and Order."

The increased security measures drew a mixed response from Iraqis — some angry over the inconvenience, others embracing any effort to stop the rampant violence.

"My friends and I who are the old women of the neighborhood went to the soldiers and welcomed them and prayed that God would help them to defeat the terrorists," said Um Sabah of the Mashtaal area in eastern Baghdad. "Although, the presence of army and vehicles is not very comfortable, we welcome it because it is for the sake of Iraq."

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

Some people left their 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, is an ally of al-Sadr, and his failure to confront the sectarian violence carried out by the cleric's Mahdi 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.

Conflicting reports, meanwhile, emerged about al-Sadr's whereabouts. The chief U.S. military spokesman, Maj. Gen. William Caldwell, said the cleric had left the country and was believed to be in Iran, but al-Sadr's supporters insisted he was still in Iraq. Al-Sadr commands a following of tens of thousands, and his influence could sway the delicate political balance in Baghdad.

Underscoring the dangers as the U.S. steps up its presence in the capital, Entifadh Qanbar, the uncle of a kidnapped Iraqi-American soldier, said a Shiite militant group had released a video to prove the missing soldier was alive. The U.S. government has offered a $50,000 reward leading to the recovery of Iraqi-born American Army translator Ahmed Qusai al-Taayie, a 41-year-old reserve soldier who was abducted by gunmen on Oct. 23.

The operation in the northeastern Baghdad was expected to take several days before troops from the 82nd Airborne Division move in with Iraqi forces to occupy the area, one of 10 districts being targeted by the military action aimed at stopping the violence that has threatened to tear the country apart. President Bush has committed 21,500 more Americans to a force that is expected to involve a total of 90,000 Iraqi and U.S. soldiers.

But Baghdad residents had seen little evidence of the new measures until Wednesday, a day after the Iraqi commander, 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.

Staff Sgt. Michael James, 32, of Chillicothe, Mo., said the area in northeastern Baghdad had been targeted before but not in such force.

"This is the final clearing. We're trying to hit all the major hotspots. I don't think it has ever been cleared as fully as it will be today," said James, of the 3rd Stryker Brigade, Combat Team, 2nd Infantry Division.

James said he wasn't surprised that the troops did find more as they hunkered down in a so-called Joint Security Station in the area for the night.

"It's never clear. These guys are going to have safe-houses all over the place. Whenever we come into one area, I'm sure they just move on," he said. "Just our presence alone is enough to push the bad guys out. They're not stupid enough to fight an entire battalion, because they will lose."

Senior military officers also appeared throughout the area to explain the operation to the Iraqis as the troops papered car windows and building facades with purple stickers listing telephone numbers and an e-mail address where they can send intelligence tips.

Bestoon Abdul Kadder, 23, an aspiring pop artist, was pessimistic about the Iraqis' ability to overcome the violence.

"Iraqi people are so bad. They just do not want to work together. I think it'll take 10 years before things will change," he said in English.

The increased security measures drew a mixed response from Iraqis — some angry over the inconvenience, others embracing any effort to stop the rampant violence.

"My friends and I who are the old women of the neighborhood went to the soldiers and welcomed them and prayed that God would help them to defeat the terrorists," said Um Sabah of the Mashtaal area in eastern Baghdad. "Although, the presence of army and vehicles is not very comfortable, we welcome it because it is for the sake of Iraq."