Military: U.S. launches 'major operation' in Afghanistan

(CNN) - U.S. troops have launched a "major operation" against Taliban fighters in southern Afghanistan, U.S. military officials in Afghanistan announced early Thursday.

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. Marines 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 told CNN 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. More than 30 U.S., British and Danish troops have been killed there since January.

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 8-year-old 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."

– CNN Pentagon Correspondent Chris Lawrence contributed to this report.