US Marines head for Camp Dwyer in Helmand province, Afghanistan. (AFP)

Garmsir (Afghanistan), July 2 (Reuters): Thousands of US Marines stormed deep into Taliban territory in an Afghan river valley today, launching the biggest military offensive of Barack Obama’s presidency.

The Marines say Operation Khanjar, or Strike of the Sword, will be decisive and is intended to seize virtually the entire lower Helmand river valley, the heartland of the Taliban insurgency and the world’s biggest opium poppy producing region.

The US military said later today that a soldier had been kidnapped in southeastern Afghanistan, before the operation in Helmand began. Kidnappings by Islamist militants were common during the Iraq war but are relatively rare in Afghanistan.

A senior Taliban commander, Mullah Sangeen, said by telephone from an undisclosed location, said the soldier was taken as a patrol walked out of its base in Paktika province. The soldier would be held until Taliban fighters held by US forces were released, he said.

In swiftly seizing the valley and holding ground there, commanders hope to accomplish within hours what overstretched Nato troops had failed to achieve over several years, and help secure Afghanistan for an August 20 presidential election after years of stalemate.

“The intent is to go big, go strong and go fast, and by doing so we are going to save lives on both sides,” Brigadier-General Larry Nicholson, commander of the Marines in southern Afghanistan, told his staff before the operation.

Violence in the Taliban-led insurgency is at its highest since the Taliban’s ouster in 2001.