NAWA, Afghanistan — Thousands of U.S. Marines poured from helicopters and armored vehicles into Taliban-controlled villages in southern Afghanistan today in the first major operation under President Barack Obama's strategy to stabilize the country.

The offensive was launched early today in Helmand province, a Taliban stronghold and the world's largest opium poppy-producing area. The goal is to clear insurgents from the hotly contested region before the nation's Aug. 20 presidential election.

It came as U.S. military announced that one of its soldiers was captured Tuesday by insurgents in eastern Afghanistan. The missing soldier was not involved in the Helmand operation.

Officials described the offensive — dubbed "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 neighboring Kandahar province.