OUTSKIRTS OF MARJAH, Afghanistan » U.S. Marines and Afghan troops faced rocket and heavy machine-gun fire from insurgents entrenched inside a Taliban-held town today, as a long-expected offensive began to re-establish government control.

The assault on Marjah is the biggest offensive since the 2001 U.S.-led invasion of Afghanistan and will serve as a major test of a new NATO strategy focused on protecting civilians. The attack is also the first major combat operation since President Barack Obama ordered 30,000 U.S. reinforcements here in December to try to turn the tide of the war.

Detecting multiple layers of insurgent defenses encircling the city, Cobra helicopters fired Hellfire missiles at tunnels, bunkers and other defensive positions. Militants also flooded the main canal at the town's entrance. Punching their way through the insurgent defenses, the ground forces reached the main canal.

Marine commanders had said they expected between 400 and 1,000 insurgents—including more than 100 foreign fighters—to be holed up in Marjah, a town of 80,000 people in Helmand province. Marjah is the biggest southern town under Taliban control and the linchpin of the militants' logistical and opium smuggling network.

Helicopters carrying hundreds of U.S. Marines and Afghan troops swooped into town under the cover of darkness early today with a ground assault of thousands of additional forces expected to follow.

“;The first wave of choppers has landed inside Marjah. The operation has begun,”; said Capt. Joshua Winfrey, commander of Lima Company, 3rd Battalion, 6th Marines, which was at the forefront of the attack.

The Kaneohe-based 1st Battalion, 3rd Marine Regiment, is part of the Marjah operation, according to reports from Marines in the field, said Marine Corps Base Hawaii spokesman Maj. Alan Crouch. About 1,100 Kaneohe Marines deployed to Afghanistan, but it is unknown how many are in Marjah.

The operation, code-named “;Moshtarak,”; or “;together,”; was described as the biggest joint operation of the Afghan war. Maj. Gen. Nick Carter, commander of NATO forces in southern Afghanistan, said 15,000 troops were involved, including some 7,500 troops fighting in Marjah.

To the north, British, American and Canadian forces struck in the Nad Ali district in a push to break Taliban power in Helmand, one of the major battlefields of the war.

In a village north of Marjah, residents said they heard gunfire before dawn, but then it went quiet. Abdul Manan, a farmer in the village of Saipo, said he finally decided to risk going out of his house and saw American troops walking by. They told him to go back inside.

Taliban militants who had been in his village two days ago disappeared. “;I don't see any Taliban now. I see Americans,”; he said by phone.

Once the town is secured, NATO hopes to rush in aid and restore public services in a bid to win support among the estimated 125,000 people who live in Marjah and surrounding villages. The Afghans' ability to restore those services is crucial to the success of the operation and to prevent the Taliban from returning.

Tribal elders have pleaded for NATO to finish the operation quickly and spare civilians—an appeal that offers some hope the townspeople will cooperate with Afghan and international forces once the Taliban are gone.

At the Pentagon a senior U.S. official said Afghan President Hamid Karzai had signed off on the attack.

Another defense official said Karzai was informed of planning for the operation well in advance. The official said it marked a first in terms of both sharing information prior to the attack and planning collaboration with the Afghan government.

Both officials spoke on condition of anonymity because there were not authorized to speak publicly.

The Marjah offensive involves close combat in extremely difficult terrain, the second official said. A close grid of wide canals dug by the United States as an aid project decades ago makes the territory a particularly rich agricultural prize but complicates the advance of U.S. forces.

On the eve of the attack, cars and trucks jammed the main road out of Marjah as hundreds of civilians defied militant orders and fled the area. For weeks, U.S. commanders had signaled their intention to attack Marjah in hopes that civilians would seek shelter.

Residents said this week that Taliban fighters were preventing them from leaving, warning the roads were planted with bombs to slow the NATO advance.

Still, many people fled anyway for the provincial capital of Lashkar Gah, 20 miles to the northeast.