Capitalizing on a lull in fighting Tuesday, hundreds of U.S. Marines pushed through a lawless region on the Syrian frontier after intense battles along the Euphrates River (search) with well-armed militants fighting from basements, rooftops and sandbag bunkers.

Insurgents kidnapped the provincial governor as a bargaining chip.

Iraq's foreign minister, meanwhile, told the Associated Press that some of Iraq's neighbors have become unnerved by the American-backed attempt to establish a robust democratic government in Baghdad and still are not doing enough to stop militants from trying to undermine the newly elected government.

As many as 100 insurgents were killed in the first 48 hours of Operation Matador (search), as American troops cleared villages along the meandering Euphrates then crossed in rafts and on a pontoon bridge, the U.S. command said. Many of the dead remained trapped under rubble after attack planes and helicopter gunships pounded their hideouts.

At least three Marines were reported killed and 20 wounded during the first three days of the offensive — the biggest U.S. operation since Fallujah (search) was taken from extremists six months ago.

The operation was launched after U.S. intelligence showed followers of Iraq's most wanted terrorist, Abu Musab al-Zarqawi (search), took refuge in the remote desert region — a haven for smugglers and insurgent suppliers. The fighters were believed to have fled to Anbar Province after losses in Iraqi cities.

After intense fighting with militants entrenched on the south bank of the Euphrates River early in the operation, Marines saw only light resistance Tuesday and advanced through sparsely populated settlements along a 12-mile stretch to the border with Syria, according to a Chicago Tribune reporter embedded with the assault, James Janega.

Residents reached by telephone in the area reported some fighting Tuesday in Obeidi and the two nearby towns of Rommana and Karabilah. They said frightened residents were taking advantage of the relative lull to flee the Qaim area.

Adel Izzedine left on foot with his wife and three children, walking six miles through farm fields to reach a village where the family caught a taxi and drove 43 miles to Rawa, east of the fighting.

"There are gunmen in the city, but there are also a lot of innocent civilians," said Izzedine, who was looking for a mosque or a school in which to spend the night. "We are living the same misery that Fallujah lived some time ago."

Gunmen kidnapped Anbar's governor Tuesday morning and told his family he would be released only when U.S. forces withdrew from Qaim, the town 200 miles west of Baghdad where the offensive began late Saturday. Gov. Raja Nawaf Farhan al-Mahalawi was seized as he drove from Qaim to the provincial capital of Ramadi, his brother, Hammad, told The Associated Press.

Lt. Col. Steven Boylan, a spokesman for U.S. forces in Iraq, said: "We don't respond to insurgent or terrorist demands."

Hoshyar Zebari, the Iraqi foreign minister, said Syria, Saudi Arabia, Kuwait, Jordan and Turkey all could do more to keep foreign fighters from entering Iraq or to prevent the insurgents from obtaining funding.

"Generally, they (Iraq's neighbors) don't like what we are doing to build democracy in that part of the world," Zebari said on the sidelines of the first Summit of Southern American-Arab countries in Brasilia, Brazil. "There is some tolerance for these terror networks on the part of the neighboring countries."

At the Pentagon, Marine Lt. Gen. James T. Conway, director of operations for the Joint Chiefs of Staff, told reporters Tuesday that the assault in the northern Jazirah Desert had run into well-equipped and trained fighters.

"There are reports that these people are in uniforms, in some cases are wearing protective vests, and there's some suspicion that their training exceeds what we have seen with other engagements further east," he said.

Marine commanders in the field told The Chicago Tribune that militants put up an unexpectedly intense fight in villages dotting the Euphrates as it snakes across the desert toward the Syrian border.

As troops erected a pontoon bridge Sunday, mortar fire began to fall on them from the nearby town of Obeidi, 185 miles west of Baghdad, the Tribune said.

Navy and Marine F/A-18 Hornet strike jets strafed the tree line and Marine Cobra attack helicopters fired rockets into insurgent hideouts, the Tribune said.

When Marines entered the town Sunday, they found insurgents prepared for battle. Sandbag bunkers stood in front of some houses, and other gunmen fired from rooftops and balconies, according to a Los Angeles Times reporter also embedded with the troops. As fighting continued into Monday, the insurgents used boats to ferry weapons across the river.

At one point, the paper said, a Marine walked into a house and a fighter hiding in the basement fired through a floor grate, killing him. Another Marine suffered shrapnel wounds when an insurgent threw a grenade through the window of a house where he was retrieving a wounded comrade, the Times said.

Insurgents attacked a Marine convoy late Monday near a U.S. base in Qaim with small arms, rocket-propelled grenades, roadside bombs and two suicide car bombs, a Marine spokesman, Capt. Jeffrey Pool, said. One explosion damaged a Humvee, and a suicide car bomber was destroyed by a Marine tank. No Marines were killed and 10 insurgents surrendered in the incident, Pool said.

Intelligence reports indicated insurgents were using the region, a known smuggling route, as a staging area where foreign fighters cross into Iraq from Syria and receive weapons and equipment for attacks in Baghdad, Ramadi, Fallujah, Mosul and other cities, Pool said.

Syria has said it is arresting would-be infiltrators and doing what it can to control the border with Iraq.

The U.S. offensive comes amid a surge of militant attacks that have targeted the U.S. military and Iraqi security forces and civilians, since the country's first democratically elected government was announced April 28.

At least two car bombs exploded Tuesday in downtown Baghdad, targeting U.S. and Iraqi troops. At least nine Iraqis were killed and 19 wounded, the Interior Ministry said. One of the bombs wounded three American soldiers, a U.S. military spokeswoman, Capt. Kelly Lewis, said.

Also Tuesday, Iraq's parliament appointed a 55-member committee of legislators from the country's Shiite, Sunni and Kurdish groups to draft a new constitution. Political leaders spent the first three months after landmark Jan. 30 elections forming a government and now have until Aug. 15 to complete the charter, which would then be voted on in a national referendum.