UPDATE 3-U.S. Marines launch assault in S.Afghan valley

* Attempt to break Taliban hold on opium-growing valley

* Marines are part of big troop build-up

* Operation marked by speed and size of assault

(Adds details from Marines statement paragraphs 4-5)

By Peter Graff

LOWER HELMAND RIVER VALLEY, AFGHANISTAN, July 2 (Reuters) - U.S. Marines launched a helicopter assault early on Thursday 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.