Only a few hundred American troops are policing the southern border of one of Afghanistan's major smuggling areas, leaving open a vast expanse of desert that the Taliban use to shuttle in weapons and fighters from Pakistan.

This dusty hamlet 75 miles north of the border in Helmand province was the Taliban's key transit point from Pakistan before the Marines arrived in July. Since then, the Marines have set up a series of patrol bases east and west of Khan Neshin to disrupt the Taliban's supply lines.

But the battalion deployed at only about 50 percent of its authorized strength, and one of its three companies is posted in central Helmand. That leaves several hundred Marines to cover roughly 6,000 square miles — an area larger than Connecticut.

As a result, the Marines may have trouble curbing Taliban supply lines as thousands of fresh troops pour into the province as part of President Barack Obama's surge.

"I would like to push closer to the border, but I can only go as far as I can support," said Lt. Col. Michael Martin, commanding officer of 4th Marine Division, 4th Light Armored Reconnaissance Battalion.

"Like Napoleon, you don't want to overextend your capabilities, or you will get your butt handed to you," said Martin, whose troops are spread out among a handful of patrol bases along the Helmand River, marking the coalition's most southern presence in the province.

Some 8,500 additional Marines are slated to arrive in Helmand by mid-2010 as part of the 30,000-troop buildup. But any decision to send more Marines south to patrol the largely uninhabited border area would leave fewer troops for the major population centers farther north.

Many Taliban fighters fled to Pakistan following the U.S.-led invasion of Afghanistan in 2001 and found sanctuary in the mountainous belt that runs between the two countries. Obama has pressed Pakistan to target the militants, but many analysts believe the government has resisted because the Taliban could serve as useful proxies if the coalition effort in Afghanistan fails.

That leaves the Marines with the difficult task of disrupting the flow of Taliban fighters into Afghanistan largely without Pakistani help.

"We are trying to make it as difficult as possible for the Taliban to stay connected to their sanctuary in Pakistan," said Capt. Timothy Newkirk, executive officer of 4th LAR's Bravo Company, which is based in a 200-year-old mud fort in the town of Khan Neshin.

Tribal elders attending a recent meeting at the Marines' most eastern patrol base reported scores of Taliban fighters flowing through a bazaar about 12 miles (20 kilometers) to the northeast near the town of Sar Banader.

"If they can get there, they can get into Marjah and they have basic freedom of movement there," Newkirk said.

Marjah is the Taliban's principal stronghold in central Helmand and will likely be a key target once the 11,000 Marines currently in the province are bolstered with the surge troops.

If the Taliban are able to send reinforcements to Marjah from Pakistan, it could make it more difficult for the Marines to take the city.

Martin, the battalion commander, said his ultimate goal would be to push south with Afghan border police the Marines are training to set up an outpost near Bahram Chah, a town just north of the Pakistani border teeming with Taliban fighters and drug smugglers making their way into Afghanistan.

But that would require hundreds of additional troops so that the Marines could extend their security control far enough south to protect any outpost near the border.

"I can drive my vehicle down to the border and back, but if I have a problem, I can't be reinforced," Martin said.

The Marines said they don't want to make the same mistake as the Army, which set up a series of remote bases near Afghanistan's eastern border with Pakistan that were constantly in danger of being overrun because the military didn't control the surrounding area. Some of those outposts have now been abandoned.

"A presence on the border would be better, but it is so far south that supporting it wouldn't be feasible right now," Newkirk said. "It wouldn't be diligent to have hundreds of Marines down in hostile territory 125 miles from the nearest medical facilities."