Heliborne U.S. and Iraqi troops pressed their sweep through a 100-square-mile swath of central Iraq on Friday in a bid to break up a center of insurgent resistance, the U.S. military said. No resistance or casualties were reported.

"We believe we achieved tactical surprise," Lt. Col. Edward Loomis, spokesman for the 101st Airborne Division, said of the day-old Operation Swarmer, the biggest air assault here in three years. He said about 40 suspects were detained, 10 of whom were later released.

In tense Baghdad, meanwhile, drive-by gunmen targeting streams of Shiite Muslim pilgrims killed three people and wounded five in Sunni areas of the city, police reported.

Devout Shiites headed south to the holy city of Karbala for a religious holiday, a pilgrimage that authorities feared would present "soft" targets in the continuing Sunni-Shiite violence roiling Iraq.

At least seven people were reported killed in scattered violence in and near Baghdad. A U.S. soldier was shot and killed in Samarra on March 16 but the military said he was not part of Operation Swarmer.

A standoff between the Shiite majority and Sunni minority underlies the political impasse blocking formation of a new government of national unity here. An all-party meeting was scheduled for later Friday to try to move those negotiations forward.

The joint U.S.-Iraqi air assault Thursday focused on a 10-by-10-mile area some 60 miles north of Baghdad and northeast of the city of Samarra, where an insurgent bombing on Feb. 22 badly damaged a major Shiite shrine, an attack that ignited days of sectarian bloodshed across Iraq in which more than 500 people died.

Fifty U.S. transport and attack helicopters ferried in and gave cover to some 1,500 U.S. and Iraqi troops taking part in Operation Swarmer — units of the 101st Airborne Division and the Iraqi 4th Division.

On Friday morning, Loomis said, the forces "continue to move" through the area. "Approximately 40 suspected insurgents were detained without resistance," he said. "Tactical interviews began immediately, and 10 detainees have been released."

The sweep also uncovered six weapons caches, the U.S. military spokesman said.

The operation was aimed at disrupting "terrorist activity in and around Samarra, Adwar and Salahuddin province," he said, an area that was a stronghold of Sunni support for Saddam Hussein's ousted Baathist party regime.

Saddam's former No. 2, Izzat Ibrahim, who was deputy chairman of the ruling Revolutionary Command Council, was from the city of Adwar and is still at large — at times thought to remain in that area.

The deputy governor of Salahuddin province, Abdullah Hussein, told reporters Friday that 48 alleged insurgents had been detained, men accused of bombings and kidnappings.

He said intelligence indicated about 200 insurgents were in the area, including people linked to the Baathist group Jaish Muhammad — Muhammad's Army — and to the al-Qaida in Iraq terror group, led by Abu Mussab al-Zarqawi.

The sweep was aimed particularly at capturing two local leaders of the Zarqawi group, said a police official. He said they had not yet been located.

Iraqi officials said Salahuddin province became more important as an insurgent center after the U.S. offensive that seized the resistance stronghold of Fallujah in late 2004, and subsequent U.S.-Iraqi offensives in other western areas close to the Syrian border.

Friday's Baghdad bloodshed began as groups of Shiite faithful, many parents with children in tow, trekked down city streets in the morning, headed for the southbound highway and Karbala, a shrine city 50 miles south of here.

At about 7:30 a.m., a BMW sedan driving alongside pilgrims in the western district of Adil opened fire, killing three and wounding two, said police Lt. Thair Mahmoud. Police later reported a second incident, also in western Baghdad, in which armed men riding in a car fired on pilgrims near Um al-Tuboul Square, wounding three.

Such attacks were feared this pilgrimage weekend as Sunni-Shiite tensions heighten across the strife-torn country. To help guard against violence in Shiite holy cities, the U.S. military dispatched a fresh battalion of the 2nd Brigade, 1st Armored Division, about 700 troops, to Iraq from its base in Kuwait to provide extra security.

Tens of thousands of devout Shiites are converging on Karbala for Monday's celebration of Arbaeen, marking the end of the 40-day mourning period after the date of the death of Imam Hussein, the Prophet Muhammad's grandson, killed in Karbala in 680 A.D.

In other violence:

— A bomb left on a minibus exploded at midday Friday and killed two passengers and wounded four in a Shiite district of Baghdad, police reported.

— Police in a Shiite area of east Baghdad late Thursday found the bodies of four Sunni men who had been seized from a taxi by masked gunmen the day before in western Baghdad.

— Six mortar rounds landed on six houses Friday in a mixed Sunni-Shiite area of Khan Bani Saad, 10 miles north of Baghdad, killing one person and wounding three, police reported.

Iraq's new Parliament held its first session on Thursday, as the first permanent elected legislature since the U.S. invasion, which began three years ago this coming Monday.

The lawmakers immediately adjourned, however, after taking their oaths of office, since the deep-seated sectarian disputes have all but paralyzed efforts to name a prime minister and Cabinet. The U.S. ambassador, Zalmay Khalilzad, has been trying to broker talks to establish a government embracing major factions in a way acceptable to Shiite, Sunni and Kurdish blocs in Parliament.