Thousands of U.S. marines and hundreds of Afghan troops moved into Taliban-controlled villages with armour and helicopters early Thursday in the first major operation under U.S. President Barack Obama's revamped strategy to stabilize Afghanistan.

The offensive was launched shortly after 1 a.m. local time Thursday in Helmand province, a Taliban stronghold in the southern part of the country and the world's largest opium poppy producing area.

The goal is to clear insurgents from the hotly contested Helmand River Valley before the nation's Aug. 20 presidential election.

Dubbed Operation Khanjar, or "Strike of the Sword," the military push was described by officials as the largest and fastest-moving of the war's new phase, involving nearly 4,000 of the newly arrived marines and 650 Afghan forces.

British forces last week led similar, but smaller, missions to fight and clear out insurgents in Helmand and neighboring Kandahar provinces.

"Where we go, we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces," U.S. Marine Corps Brig. Gen. Larry Nicholson said in a statement.

Southern Afghanistan is a Taliban stronghold but also a region where Afghan President Hamid Karzai is seeking votes from fellow Pashtun tribesmen.

The Pentagon is deploying 21,000 additional troops to Afghanistan in time for the election and expects the total number of U.S. forces there to reach 68,000 by year's end. That is double the number of troops in Afghanistan in 2008, but still half as much as are now in Iraq.

Capt. Bill Pelletier, a spokesman for the marines, said the troops involved in the Thursday operation were sent in by a mixture of aircraft and ground transport under the cover of darkness.

The operation is aimed at putting pressure on insurgents, "and to show our commitment to the Afghan people that when we come in, we are going to stay long enough to set up their own institutions," Pelletier said.