LOWER HELMAND RIVER VALLEY, AFGHANIS-TAN, (Reuters) -  U.S. Marines launched a helicopter assault early yesterday in  the lower Helmand river valley in southern Afghanistan,  spokesman Capt. Bill Pelletier said.

A Reuters correspondent in the valley saw flares in the sky  over the town of Nawa, south of the provincial capital Lashkar  Gah.

Nearly 4,000 Marines and U.S. sailors are taking part in the assault, code-named Operation Khanjar (Strike of the Sword),  along with about 650 Afghan troops and police, a Marines press  statement said.

“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 …” it quoted  Brigadier General Larry Nicholson, commanding officer of the  Marine Expeditionary Brigade-Afghanistan, as saying.

The valley of irrigated wheat and opium fields along the  Helmand river is largely in the hands of Taliban fighters who  have resisted British-led NATO forces for years.

The United States has sent 8,500 Marines to Helmand province  in the last two months, the largest wave of a massive buildup of  forces that will see the number of U.S. troops in Afghanistan  rise from 32,000 at the beginning of this year to 68,000 by  year’s end.

President Barack Obama has declared the Taliban insurgency  in Afghanistan and neighbouring Pakistan to be the main security  threat facing the United States.

Helmand province is one of the Taliban’s main heartlands in  southern Afghanistan and produces the largest share of the  country’s opium crop which supplies 90 percent of the world’s  heroin. Attacks by Taliban fighters are at their highest levels  since the strict Islamists were driven out of Kabul by  U.S.-backed Afghan opponents in 2001 after refusing to turn over  Osama bin Laden in the wake of the Sept. 11 attacks on the  United States.

U.S. and NATO commanders have said they intend to deploy  American reinforcements to seize Taliban-held territory in the  south in time for Afghanistan to hold a presidential election on  Aug. 20.