Thousands of US Marines stormed into an Afghan river valley by helicopter and land early today, launching the biggest military offensive of Barack Obama's presidency with an assault deep into Taliban territory.

Operation Khanjar, which the Marines call simply "the decisive op", is intended to seize virtually the entire lower Helmand River valley, heartland of the Taleban insurgency and the world's biggest heroin producing region.

In swiftly seizing the valley, commanders hope to accomplish within hours what Nato troops had failed to achieve over several years, and by doing so turn the tide of a stale-mated war in time for an Afghan presidential election on August 20.

"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 Brigadier General Larry Nicholson, commander of the Marines in southern Afghanistan said in a statement.

He told his staff before the operation: "The intent is to go big, go strong and go fast, and by doing so we are going to save lives on both sides."

The operation would have an initial highly aggressive stage lasting 36 hours, AFP reported.

Wave upon wave of helicopters landed Marines in the early morning darkness at locations throughout the valley, a crescent of opium and wheat fields criss-crossed by canals and dotted with mud-brick homes, where firmly entrenched fighters defied Nato forces for years.

Hundreds more Marines raced by ground in convoys through a barren area known as the Desert of Death.

In all, about 4,000 Marines surged forward and thousands more were mobilised to assist them along with 650 Afghan forces, an operation by foreign ground troops on a scale unseen in Afghanistan since the Soviet withdrawal in 1989.

Captain Bill Pelletier, a spokesman for the Marines, said no clashes with Taleban troops had been reported, but a Marine had been slightly injured by a roadside bomb.

The operation was 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," Captain Pelletier said.

He said the US military was prepared for casualties, but stressed that "it is absolutely essential that no civilians be harmed".

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end