BAGHDAD, 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 terrorists since the weekend operation began.

The American 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.

American 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 American offensives since the insurgent stronghold of Fallujah fell last fall.

Meanwhile, terrorists claimed in an Internet posting to have captured a Japanese man after ambushing a international British security company’s convoy in western Iraq. A spokesman for the company confirmed the employee was missing.

It wasn’t clear whether Sunday’s attack was related to the American operation, which was about 80 miles away.

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, a Marine spokesman, Captain Jeffrey Pool, said.

A senior military official in Washington said the offensive was targeting followers of the leader of Al Qaeda in Iraq, Abu Musab al-Zarqawi, 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,” a spokesman for American forces in Iraq, Lieutenant Colonel Steven Boylan, said. “That’s a fairly porous area of the border because of the terrain. It is very difficult.”

Acting on information from a captured associate of Mr. al-Zarqawi, American 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, Captain 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,” Colonel 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 American forces.

Frightened residents 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 push comes amid a barrage of insurgent 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 and wounding 15, police said.

The militant group Ansar al-Sunnah Army said it had taken a Japanese citizen, 44-year-old Akihito Saito, as a hostage, posting a photocopy of his passport, including his picture, on the group’s Web site.

The group claimed its fighters ambushed a convoy of five foreign contract workers protected by a dozen Iraqi security men near Hit, 90 miles west of Baghdad. It claimed all were killed in the fight except for Mr. Saito, who it said was “severely injured.”

An ID card posted on the Internet identified Mr. Saito as a security manager for Hart GMSSCO, a Cyprus-based security firm. The CEO of Hart, Simon Falkner, said in London that there was an ambush with casualties Sunday night involving Hart personnel.

Later, a spokesman for Hart said Mr. Saito was among a number of the company’s workers who were unaccounted for after the attack.

“We can confirm that a number of Hart personnel are unaccounted for and one of them is Mr. Saito,” the spokesman said, speaking on condition of anonymity.

The Kyodo news agency in Japan said a senior Foreign Ministry official quoted Mr. Saito’s parents as saying the man in the picture looked like their son.

The family of another hostage, Australian national Douglas Wood, offered yesterday to make a generous “charitable donation” to the people of Iraq to secure his freedom. Australia’s top Muslim cleric was traveling to Baghdad to try to negotiate his release.

Militants who kidnapped the 63-year-old California resident released a video Friday demanding that Australia start pulling its troops out of Iraq within 72 hours. The deadline passed yesterday with no word of his fate.

Iraq’s new Shiite Arab premier had hoped to dent support for the escalating insurgency by including members of the disaffected Sunni Arab minority in his new government.

Five new Cabinet members were sworn in yesterday, four of them Sunnis. But a Sunni selected to be human rights minister turned down the job Sunday, saying his appointment was merely due to tokenism.