U.S. forces are stepping up their operation against Taliban insurgents in southern Afghanistan. It's the biggest offensive yet for President Barack Obama.

The military said nearly 4,000 Marines and U.S. soldiers are taking part in the assault in the lower Helmand River Valley, along with about 650 Afghan troops and police.

The valley of irrigated wheat and opium fields, known as the world's biggest heroin-producing region, is largely in the hands of Taliban fighters who have resisted British-led NATO forces for years.

The U.S. has sent 8,500 Marines to Helmand Province in the last two months, with the total now at 10,000.

This is the largest wave of a massive buildup that will see U.S. troops in Afghanistan rise from 32,000 in January to 68,000 by year's end.

President Obama has declared the Taliban insurgency in Afghanistan and neighboring Pakistan as the main security threat facing the United States.

U.S. and NATO commanders intend to deploy reinforcements to seize Taliban-held territory in the south in time for Afghanistan to hold its presidential election on August 20.