OUTSIDE GARMSER, Afghanistan — U.S. Marines exchanged gunfire with militants today after pouring into a Taliban-held town in southern Afghanistan in the first major American operation in the region in years.

Several hundred Marines, many of them veterans of the conflict in Iraq, pushed into the town of Garmser in pre-dawn light in an operation to drive out the insurgents, stretching NATO's presence into an area littered with opium poppy fields and classified as Taliban territory.

U.S. commanders say Taliban fighters were expecting an assault and planted homemade bombs in response. The British have a small base on the town's edge but Garmser's main marketplace is closed because of the Taliban threat.

Marines moved into town by helicopter and Humvee for Tuesday's assault in the southern province of Helmand, the first major task undertaken by the 2,300 Marines in the 24th Marine Expeditionary Unit, which arrived last month from Camp Lejeune, North Carolina, for a seven-month deployment. Another 1,200 Marines arrived to train Afghan police.

Maj. Tom Clinton, the American commander at Forward Operating Base Dwyer, a British outpost 10 miles west of Garmser, said militants and Marines exchanged fire in two parts of Garmser on Tuesday. There was no immediate word on casualties.

"We haven't seen anybody who isn't carrying a gun," Clinton said of the mostly deserted town. "They're trying to figure out what we're doing. They're shooting at us, letting us know they're there."

Clinton, 36, of Swampscott, Mass., said Marines had also found bomb-making material and rockets in town. He said he was worried about the possibility of attacks using homemade bombs.

The Marines' mission is the first carried out by U.S. forces this far south in Helmand province in years. An operation late last year to take back the Taliban-held town of Musa Qala on the north end of Helmand involved U.S., British and Afghan forces.

Helmand province is the world's largest opium poppy growing region and has been a flash point of the increasingly violent insurgency in the last two years. British troops — who are responsible for Helmand — have faced fierce battles on the north end of Helmand.

Most U.S. troops operate in the east, along the border with Pakistan, but Britain, with 7,500 troops, and Canada, with 2,500 troops in neighboring Kandahar province, have not had enough manpower to tame the south.

More than 8,000 people died in insurgency-related violence last year. Militants set off more than 140 suicide bombs. Taliban fighters have been increasingly relying on roadside bombs and suicide attacks after being routed in force-to-force battles in the past.

The Marines had prepared on Monday by cleaning weapons and handing out grenades. The leader of one of the three companies involved — Charlie Company commander Capt. John Moder — said his men were ready.

"The feeling in general is optimistic, excited," said Moder, 34, of North Kingstown, Rhode Island. "They've been training for this deployment the last nine months. We've got veteran leaders."

Many of the men in the 24th Marine Expeditionary Unit served in 2006 and 2007 in Ramadi, the capital of Anbar province in western Iraq. The vast region was once the stronghold of al-Qaida in Iraq before the militants were pushed out in early 2007.

Moder said that experience would affect how his men fight in Afghanistan. "These guys saw a lot of progress in Ramadi, so they understand it's not just kinetic (fighting) but it's reconstruction and economic development."

But on the initial assault, Moder said his men were prepared to face mines and homemade bombs and "anybody that wants to fight us."

One Marine in Charlie Company, Cpl. Matt Gregorio, 26, from Boston, alluded to the fact the Marines had been in Afghanistan for six weeks without carrying out any missions. He said the mood was "anxious, excited."

"We've been waiting a while to get this going," he said.