QAIM, Iraq — Hundreds of American troops backed by helicopter gunships and warplanes swept into remote desert villages near the Syrian border yesterday, hunting for followers of Iraq's most wanted terrorist and reportedly killing as many as 100 militants since the weekend operation began.

The U.S. military said some foreign fighters were believed among the insurgents killed in the first 48 hours of the assault, which began late Saturday in the border town of Qaim, about 200 miles west of Baghdad. At least three Marines were killed in the region, it said.

At the vanguard of the assault, Marines who swept into the Euphrates River town of Obeidi confronted an enemy they had not expected to find — and one that attacked in surprising ways.

As they pushed from house to house in early fighting, trying to flush out the insurgents who had attacked their column with mortar fire, they ran into sandbagged emplacements behind garden walls. They found a house where insurgents were crouching in the basement, firing upward through slits hacked at ankle height in the ground-floor walls, aiming at spots that the Marines' body armor did not cover.

The shock was that the enemy was not supposed to be in this town at all. Instead, American intelligence indicated that the insurgents had massed on the other side of the river. Marine commanders expressed surprise yesterday at not only the insurgents' presence but also the extent of their preparations, as if they expected the Marines to come.

"That is the great question," said Col. Stephen Davis, commander of Marine Regimental Combat Team 2, responsible for this rugged corner of Anbar province near the Syrian border. American officials describe the region, known as the al-Jazirah Desert, as a haven for foreign fighters who shuttle across the porous Syrian border, using the broken terrain for cover.

Three Marine companies and supporting armored vehicles crossed to the north side of the Euphrates early yesterday, using rafts and a hastily constructed pontoon bridge. From there they were expected to roll west toward the border, raiding isolated villages where insurgents are believed to be hiding weapons and fighters. The offensive, which has been planned for weeks, is expected to stretch for several days.

"We're north of the river [and] we're moving everywhere we want to go," Davis said late yesterday. "Resistance is predictably low, but I do not expect it to stay that way."

In recent weeks, intelligence suggested that insurgents were using the area to build car bombs that would be used in attacks in Baghdad and other cities. More than 300 Iraqis have been killed in insurgent attacks in the past two weeks, following the formation of a Shiite-dominated government.

A senior military official in Washington said the Marines were targeting followers of terrorist leader Abu Musab al-Zarqawi, who has been linked to many of the most violent attacks on U.S. and Iraqi forces.

Acting on information from a captured al-Zarqawi associate, U.S. forces moved into Qaim overnight Saturday, killing six insurgents and detaining 54 suspects, the military said in a statement. Local residents were providing a "wealth of information" about the insurgency and foreign fighters in their area, said Capt. Jeffrey Pool, a Marine spokesman.

Some residents, meanwhile, were caught in the middle.

Iraqis cowered in their homes yesterday as bombs exploded and warplanes roared overhead.

"It's truly horrific, there are snipers everywhere, rockets, no food, no electricity," said Abu Omar al-Ani, a father of three reached by telephone in Qaim. "Today, five rockets fell in front of my house ... we are mentally exhausted."

The offensive is described as one of the largest involving U.S. troops since the assault on Fallujah last fall. It involves more than 1,000 Marines, Navy and Army personnel, backed by helicopters and jet fighters.

Though military commanders in Baghdad announced that 100 insurgent fighters were killed in the early fighting, along with the three Marines, Davis' figures were lower. He said "a couple of dozen" insurgents had been killed in Obeidi, about 10 at another river crossing near Qaim, and several in airstrikes north of the river.

Other commanders said they had recovered few bodies but had seen blood trails that suggested insurgents were dragging away wounded or dead fighters.

The number of insurgents in the region is "in the hundreds," Davis said. "How many hundreds is tough to tell."

But more surprising, he said, was the insurgents' preparation and tactical prowess, a development that he said reinforced intelligence that many of the insurgents have been trained outside Iraq.

Davis described sophisticated attacks in which the detonation of a roadside bomb would be quickly followed by accurate mortar or rocket fire, then machine-gun fire as Marines raced to the area.

"They clearly have trained people," he said. "It looks rehearsed."

Marines who had captured an existing bridge over the Euphrates north of Qaim came under attack early yesterday by several insurgents, Davis said. An air assault killed about 10 of the insurgents, who were wearing flak jackets — which American officials generally take as a sign that the fighters were not local Iraqis.

As the fighting raged Sunday in Obeidi and other towns along the Euphrates, a platoon of Marines perched on cliffs near the Syrian border, hoping to call in airstrikes on any fighters who tried to slip across, commanders said.

The commanders reported that the Marines saw truckloads of men speeding toward remote houses in the region, leaping off the trucks and racing inside.

They came out carrying armloads of rocket-propelled grenades and assault rifles, loaded them onto their trucks and headed back east, toward the fighting.