CAMP LEATHERNECK, Afghanistan - Columns of U.S. Marines in eight-wheeled armored vehicles pushed deep into southern Afghanistan yesterday in an attempt to cut off Taliban supply lines from Pakistan and restore order in areas long neglected by shorthanded NATO forces.

The movement of the Marines to the town of Khan Neshin in the lower Helmand River valley is the most significant deployment of U.S. forces in areas near the Pakistani border with southern Afghanistan, and it reflects a growing concern among U.S. military and intelligence officials that much of the violence that has plagued the south is linked to a flow of fighters and munitions from Pakistan's Baluchistan region.

The drive to Khan Neshin is part of a Marine campaign to root out Taliban insurgents by restoring the authority of local government officials and police departments in the Helmand River valley. The 4,000-strong operation - one of the largest conducted by the U.S. military in Afghanistan - is intended to demonstrate new strategies advocated by the Obama administration to turn around a struggling seven-year-old war effort.

As units from the Second Marine Expeditionary Brigade fanned out on foot in other parts of the valley yesterday, their principal focus was to meet local leaders - and to set about winning their confidence - not to hunt down Taliban fighters. Marine officers distributed handbills explaining their presence and talked to residents with the help of interpreters. Some Marine companies, which had arrived by helicopter early yesterday morning, bedded down for the night in empty homes instead of constructing bases with razor wire and sand-filled barriers.

Although the troops encountered roadside bombs and small-arms attacks, which resulted in the death of one Marine, commanders opted to mute their return fire. In the first 24 hours of the operation, the Marines did not fire artillery or call for fighter planes to drop bombs.

The brigade's operations officer, Col. Eric Mellenger, said the absence of opposition in Khan Neshin represented a "major political and security success" and would allow the Marines to meet with town elders over the next few days. In other areas, he said, "people have been coming up to us with information about the Taliban."

U.S. military and diplomatic officials say that the vast majority of Afghans, even those in violence-wracked places such as Helmand province, do not want to be ruled by the Taliban and its extremist ideology. The officials contend that if Afghans are provided security and basic services, they will switch allegiances and support the local government.

Reactions to the Marine operation varied across the valley. In Khan Neshin, residents largely stayed off the streets, wary of being caught in the cross fire of possible Taliban attacks on the troops. In the northern areas, around the Nawa district, several residents approached Marines with information about where roadside bombs had been planted. Farther south, in Garmser district, a Marine company was attacked by a group of insurgents, who eventually retreated to a housing compound along a canal.

A gun battle at the house was the day's most significant combat engagement, resulting in the Marine fatality and the deaths of at least three insurgents. As the sun set, it appeared that the standoff would continue through the night.

Another significant challenge for the Marines was the 110-degree weather. Loaded down with backpacks and ammunition, and insulated by flak vests and Kevlar helmets, several fell ill from heatstroke, and five had to be evacuated for medical treatment. Helicopters had to be summoned to replenish units with extra water.

Commanders expressed surprise that the Marine battalion that moved south to Khan Neshin - an imposing collection of 70 armored vehicles, each weighing 17 tons - did not encounter more resistance.

The experience in Khan Neshin, a hardscrabble riverfront town that sits north of a vast desert stretching into Pakistan, suggests that Taliban fighters there, and elsewhere in the Helmand River valley, may be lying low to assess what the Marines do before trying to retaliate with roadside bombs and suicide attacks. But the Marine presence may also lead some of the fighters to move to other parts of the country or seek other infiltration paths from Pakistan.