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

The offensive was launched shortly after 1 a.m. Thursday local time (4:30 p.m. EDT Wednesday, 2030 GMT Wednesday) 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.

Officials described the operation - 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 neighboring 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 Brig. Gen. Larry Nicholson said in a statement.

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 the offensive. It gave no more details, but U.S. and Pakistani officials have expressed concern that stepped-up operations in southern Afghanistan could push the insurgents across the border.

Transport helicopters carried hundreds of Marines into the village of Nawa, some 20 miles (30 kilometers) south of the provincial capital of Lashkar Gah, in a region where no U.S. or other NATO troops have operated in large numbers.

The troops took many insurgents by surprise, dropping behind Taliban lines, said Capt. Drew Schoenmaker, from Greene, New York.

"We are kind of forging new ground here. We are going to a place nobody has been before," said Schoenmaker, 31, who commands Bravo Company of the 1st Battalion, 5th Marine Regiment.