BAGHDAD, Iraq — Heliborne U.S. and Iraqi troops pressed their sweep through a 100-square-mile swath of central Iraq today 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 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 today 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.

This 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.