Nawa: Thousands of US 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 US 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 south of the provincial capital of Lashkar Gah, in a region where no US 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.

Daybreak brought the sporadic crackle of gunfire. Medical helicopters circled overhead and landed, indicating possible early casualties among the Marines.

A marine unit in Nawa traded gunfire with a group of some 20 insurgents, while Afghan troops exchanged small arms fire with militants after they were attacked with rocket propelled grenades fired from several houses. A Cobra helicopter circling overhead for most of the day fired rockets at a tree-line nearby. Other troops walked through fields of corn and past mud-wall homes.

Only a handful of villagers dared to venture outside.

A roadside bomb early in the mission wounded one Marine, but he was able to continue, spokesman Capt. Bill Pelletier said.

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 elections and expects the total number of US 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 many as are now in Iraq.