(CNN) -- U.S. troops have launched a "major operation" against Taliban fighters in southern Afghanistan, U.S. military officials announced in Afghanistan early Thursday. U.S. Marines gather for a briefing in Helmand Province, Afghanistan, on Wednesday. U.S. Marines gather for a briefing in Helmand Province, Afghanistan, on Wednesday.

About 4,000 Americans, mostly from the Marines, and 650 Afghan soldiers and police launched Operation Khanjar -- "strike of the sword" -- in the Helmand River valley, the U.S. command in Kabul announced.

The push is the largest since the Pentagon began moving additional troops into the conflict this year, and it follows a British-led operation launched last week in the same region, the Marines said.

It is also the first big move since U.S. Gen. Stanley McChrystal took over as the allied commander in Afghanistan in mid-June.

In Washington, a senior defense official said the size and scope of the new operation are "very significant." "It's not common for forces to operate at the brigade level," the official said. "In fact, they often only conduct missions at the platoon level. And they're going into the most troubled area of Afghanistan."

Helmand Province, where much of the fighting is taking place, has been a hotbed of Taliban violence in recent months. At least 25 U.S. and British troops have been killed there in 2009.

The defense official said the operation is a "tangible indication" of the new approach that McChrystal -- a former chief of the Pentagon's special operations command -- is bringing to the nearly eight-year war.

"They're not just doing an offensive push to get bad guys; they're going in to hold the area and stay there," the official said. "This approach is indicative of McChrystal's philosophy: measuring success by the number of Afghans protected, not bad guys killed."

The Obama administration has moved about 21,000 U.S. troops to Afghanistan, the original front in the war launched after the September 11 attacks.

During his confirmation hearing in June, McChrystal told senators that the conflict requires a new focus on counterinsurgency to reduce violence and build support for the U.S.-led NATO alliance among Afghans.

"Although I expect stiff fighting ahead, the measure of success will not be enemy killed. It will be shielding the Afghan population from violence," he said.

The Islamic fundamentalist Taliban ruled most of Afghanistan before its allies in the al Qaeda terrorist network attacked New York and Washington in 2001. Though quickly toppled after the attacks, its leaders escaped, and the movement regrouped in the Afghan countryside and across the border in Pakistan.

U.S. Marines Try to Retake Afghan Valley From Taliban
By RICHARD A. OPPEL Jr.
Published: July 1, 2009

KABUL, Afghanistan — Almost 4,000 United States Marines, backed by helicopter gunships, pushed into the volatile Helmand River valley in southwestern Afghanistan early Thursday morning to try to take back the region from Taliban fighters whose control of poppy harvests and opium smuggling in Helmand provides major financing for the Afghan insurgency.

The Marine Expeditionary Brigade leading the operation represents a large number of the 21,000 additional troops that President Obama ordered to Afghanistan earlier this year amid rising violence and the Taliban’s increasing domination in much of the country. The operation is described as the first major push in southern Afghanistan by the newly bolstered American force.

Helmand is one of the deadliest provinces in Afghanistan, where Taliban fighters have practiced sleek, hit-and-run guerrilla warfare against the British forces based there.

British troops in Helmand say they rarely get a clear shot at Taliban attackers, who ambush them with improvised explosive devices, rocket-propelled grenades and assault rifles. The explosive devices — some made with fertilizer distributed to Afghan farmers in an effort to wean them from opium production — are the most feared weapon. The Taliban favor ambushes in the morning and evening and do not often strike during the blazing afternoon heat.

In recent weeks some British troops have been setting up what are known as “blocking positions” on bridges over irrigation canals and at other locations, apparently to help stop the flow of insurgents during the main military operation and to establish greater security before the presidential election scheduled for August. The British forces, whose main base in Helmand is adjacent to the main Marine base, will continue to support the new operation.

The British have had too few troops to conduct full-scale counterinsurgency operations and have often relied on heavy aerial weapons, including bombs and helicopter gunships, to attack suspected fighters and their hideouts. The strategy has alienated much of the population because of the potential for civilian deaths.

Now, the Marines say their new mission, called Operation Khanjar, will include more troops and resources than ever before, as well as a commitment by the troops to live and patrol near population centers to ensure that residents are protected. More than 600 Afghan soldiers and police officers are also involved.

“What makes Operation Khanjar different from those that have occurred before is the massive size of the force introduced, the speed at which it will insert, and the fact that where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces,” the Marine commander in Helmand Province, Brig. Gen. Larry Nicholson, said in a statement released after the operation began. The Marines will be pushing into areas where NATO and Afghan troops have not previously established a permanent presence. As part of the counterinsurgency strategy, the troops will meet with local leaders, help determine their needs and take a variety of actions to make towns and villages more secure, said Capt. Bill Pelletier, a spokesman for the Marines, according to The Associated Press.

“We do not want people of Helmand Province to see us as an enemy; we want to protect them from the enemy,” Captain Pelletier said, The A.P. reported.

The goal of the operation is to put pressure on the Taliban militants “and to show our commitment to the Afghan people that when we come in we are going to stay long enough to set up their own institutions,” he said.

The 21,000 additional American troops that Mr. Obama authorized after taking office in January almost precisely matches the original number of additional troops that President George W. Bush sent to Iraq two years ago. It will bring the overall American deployment in Afghanistan to more than 60,000 troops. But Mr. Obama avoided calling it a surge and resisted sending the full reinforcements initially sought by military commanders.

Instead, Mr. Obama chose to re-evaluate troop levels over the next year, officials said. The Obama administration has said that the additional American commitment has three main strategies for denying havens for the Taliban and Al Qaeda: training Afghan security forces, supporting the weak central Afghan government in Kabul and securing the population.

In late March, Mr. Obama warned Congressional leaders that he would need more than the $50 billion in his budget for military operations and development efforts.

Asked by lawmakers about the prospect of reconciliation with moderate members of the Taliban, officials said Mr. Obama replied that he wanted to sift out hard-core radicals from those who were fighting simply to earn money.

Eros Hoagland contributed reporting.