U.S. Marines from the 2nd Marine Expeditionary Battalion walk through the sand Monday inside Camp Leatherneck in Afghanistan’s Helmand province. New U.S. troops ordered to Afghanistan are fanning out across the dangerous south on a mission to defeat the Taliban insurgency and to change the course of a war claiming American lives at a record pace.

Camp Leatherneck, Afghanistan ? Teams of builders worked through dust storms Monday to expand a base for a brigade of U.S. Marines now fanning out across southern Afghanistan to change the course of a war claiming American lives faster than ever before.

Some 10,000 Marines have poured into Afghanistan in the last six weeks, the military said Monday, transforming this once small base in the heart of the country’s most violent province, Helmand, into a desert fortress.

The statement to embedded journalists, including a team from The Associated Press, was the first confirmation that the military has fully deployed the first wave of 21,000 additional troops President Barack Obama ordered to Afghanistan this year to help stanch an increasingly violent Taliban insurgency.

The 2nd Marine Expeditionary Brigade, normally based at Camp Lejeune, N.C., will battle the Taliban as well as train and fight alongside Afghan security forces.

“This is where the fight is, in Afghanistan,” said 1st Sgt. Christopher Watson, who like many of the troops was most recently deployed in Iraq. “We are here to get the job done.”

The United States and its allies invaded Afghanistan in late 2001 because the country’s extremist Taliban leaders were sheltering Osama bin Laden and al-Qaida, the Islamic terrorist group behind the Sept. 11 attacks.

The forces quickly defeated the Taliban, pushing the militants out of Kabul and their southern base in Kandahar. But a guerrilla war, which turned dangerously violent in 2006, has bedeviled the international coalition and Afghan government.

While the insurgency is active across much of the country, its stronghold remains in Helmand. The province is home to the world’s largest opium-poppy growing region and borders Pakistan, where commanders say the Taliban leadership supplies money and recruits.

The Taliban have become entrenched in Helmand because of a lack of international and Afghan troops. Several thousand British forces have engaged in heavy fighting in Helmand for much of the past three years. Last year, a much smaller U.S. Marine force joined them, helping to clear the town of Garmser of insurgents.

“We are not under the impression that is going to be easy,” said Capt. Bill Pelletier, a Marine spokesman. “They are an adaptive enemy.”

A dust storm whipped across Camp Leatherneck early Monday but did little to stop the pace of construction. Hard-hatted workers put up wooden structures to house command centers and dining facilities, while cranes dropped blast walls close to the rows of air-conditioned tents housing troops.

The Marines are slowly spreading out to smaller bases in an area of operations about 7,000 square miles, said Pelletier, adding there already have been several engagements with insurgents. The military has yet to announce any losses in combat suffered by the brigade.

An Army brigade of some 7,000 troops will follow this summer along with 4,000 forces to train Afghan security forces.

The surge will bring American troop levels from about 55,000 now to more than 68,000 by the end of 2009 — about half of the nearly 140,000 troops currently in Iraq.

The buildup has led to comparisons with Iraq, where an influx of troops in 2007 is credited with helping to reduce violence.

But unlike Iraq, where the U.S. plans to phase out its role by 2012, the military envisions a long-term presence in Afghanistan.

Adding troops in a country with a history of resistance to foreign forces risks increasing Afghans’ resentment, which in turn fuels the insurgency.