BAGHDAD, Iraq -- Hundreds of American troops backed by helicopter gunships and warplanes swept into remote desert villages near the Syrian border Monday, 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.

U.S. officials described the area as a known smuggling route and a haven for foreign fighters involved in Iraq's insurgency. The assault was the biggest U.S. offensive since the insurgent stronghold of Fallujah fell last fall.

The offensive was being conducted by Regimental Combat Team 2, a joint force of about 1,000 Marines, sailors and soldiers commanded by the 2nd Marine Division, and expected to last several days in an area along the Euphrates River in the al-Jazirah Desert, said Capt. Jeffrey Pool, a Marine spokesman.

A senior military official in Washington said the offensive was targeting followers of Abu Musab al-Zarqawi, leader of Al-Qaida in Iraq, who are believed operating in the remote region.

"This is an area which we believe has been pretty heavy with foreign insurgents from many different areas -- Syria, Jordan, Saudi Arabia, Palestine," said Lt. Col. Steven Boylan, a spokesman for U.S. forces in Iraq. "That's a fairly porous area of the border because of the terrain. It is very difficult."

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, Pool said.

On Sunday, troops moved into villages in and around Obeidi, a town about 185 miles west of Baghdad, and started to pushed north across the Euphrates, according to The Chicago Tribune, which has a reporter embedded with the combat team.

"Our analysis is that there's a foreign fighter flow from Syria," Col. Stephen Davis, commander of Regimental Combat Team 2, told the Tribune. "The trademark of these folks is to be where we're not. We haven't got north of the river for a while."

The newspaper quoted some Marines as saying residents of one riverside town turned off all their lights at night, apparently to warn neighboring villages of the approaching U.S. forces.

Frightened residents cowered in their homes Monday 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 push comes amid a surge of militant attacks that have killed more than 310 people since April 28, when the new Iraqi government was announced.

At least four car bombs -- including two suicide attacks -- exploded in Baghdad, killing at least five people.