Nawa: US 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. 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 US 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 US-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.
US 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 August 20.