UBAYDI, Iraq — More than 1,000 U.S. troops supported by fighter jets and helicopter gunships attacked villages yesterday along the Euphrates River, seeking to uproot a persistent insurgency in an area intelligence indicated has become a haven for foreign fighters flowing in from Syria.

Marine officials said the operation near the Syrian border, one of the largest involving U.S. ground troops since the battle for Fallujah last fall, is expected to last for several days.

Plans to press the attack north of the Euphrates were temporarily derailed when insurgents on the south side of the river launched counterattacks, sparking heavy fighting in the small river town of Ubaydi.

While some U.S. units were able to conduct limited raids north of the Euphrates yesterday, most of the rest were trapped south of the river while Army engineers struggled to build a pontoon bridge.

U.S. military officials in Baghdad said forces that crossed the Euphrates had killed six insurgents and captured 54 more, using information gleaned from a captured aide to terrorist leader Abu Musab al-Zarqawi.

Predator drones conducted air surveillance north of the river while other aircraft launched attacks on suspected insurgents.

U.S. military officials contend this region of Anbar province west of Baghdad has become a rallying spot for insurgents, who then sneak into cities as far east as Baghdad. Officials say some float back and forth across Iraq's porous border with Syria.

Yesterday's elaborate mission, planned for weeks, was designed to combat that. But a combination of bad luck and insurgent counterattacks quickly disrupted the plan.

Overnight, the Army's 814th Multi-Role Bridge Company crawled along back roads toward the Euphrates, where it was to construct a pontoon bridge that would allow the Marines to cross. The trucks were forced to use their headlights to allow them to spot land mines along the route.

But the routine safety practice apparently alerted residents to the convoy. An entire town along the route switched off its lights all at once, a move Marines believe is a signal from one river town to the next.

As the bridging unit approached the river crossing early yesterday, they switched off the truck headlights even though many soldiers lacked night-vision goggles. In the gloom, one truck rolled off the road and into a ditch, bringing the column to a dead halt.

The soldiers soon discovered another problem: The river banks, sodden after recent rains, might have been too wet to support the oncoming American tanks.

When dawn broke, the column came under mortar fire from Ubaydi, the nearest town. Two mortars dropped within feet of the Marines' command post and an officer's Humvee. The insurgents the Marines expected to find north of the river were on the south side as well.

Marines and soldiers scrambled into a ramshackle building on a bluff overlooking the river, then devised a new strategy: They would not cross the river yesterday. They would attack Ubaydi instead.

To support Marines on the ground, F/A-18 fighters strafed a treeline on the edge of town, silencing sporadic fire coming from the trees. Helicopter gunships fired rockets and machine guns into buildings in the town.

Meanwhile, Marines entered the north end of Ubaydi in tough fighting that lasted until after sunset.