( 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 . 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 . CNN Pentagon Correspondent Chris Lawrence contributed to this report .
650 Afghans troops , 650 American involved in Operation Khanjar . It focuses on Obama valley , hotbed of Operation Khanjar violence . Operation 's size and scope are `` very significant , '' official says .