A massive US military operation aimed at driving the Taliban from south Afghanistan and winning the trust of locals swung into action yesterday.

Thousands of Marines poured from helicopters and armoured vehicles into villages in Helmand province in the first major operation under President Barack Obama's strategy to stabilise the country.

Helmand is a Taliban stronghold and the world's largest opium poppy-producing area. The goal is to clear insurgents from the region before Afghanistan's presidential election on August 20.

The Marines did not suffer any serious casualties initially and saw only sporadic resistance, a spokesman said.

“The enemy has chosen to withdraw rather than engage for the most part,” he said.

The operation came as the US announced that one of its soldiers was missing and believed captured by insurgents in eastern Afghanistan on. He was not involved in the Helmand operation.

Officials described the offensive — dubbed Khanjar or “Strike of the Sword” — as the largest and fastest-moving of the war's new phase and the biggest Marine offensive since the one in Fallujah, Iraq, in 2004.

It involves nearly 4,000 newly arrived Marines, plus 650 Afghan forces. British forces last week led similar, but smaller, missions to clear out insurgents in Helmand and neighbouring Kandahar province.

“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,” Marine Corps General Larry Nicholson said.

Pakistan's army said it had moved troops from elsewhere on its side of the Afghan border to the stretch opposite Helmand to try to stop any militants from fleeing. It gave no more details, but US and Pakistani officials are concerned that increased operations in southern Afghanistan could push insurgents across the border.

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

Once on the ground, the troops will meet with local leaders, hear their needs and act on them, a Marine spokesman said.

“We do not want the people of Helmand province to see us as an enemy. We want to protect them from the enemy,” he said.

Thousands of British forces, fighting under Nato command, have been in Helmand since 2006 with broadly the same strategy, but security has deteriorated.

They met with stronger resistance than expected against Taliban funded by the opium and heroin trade.

Reversing the insurgency's momentum has been a key component of the new US strategy.