Operation Khanjar in Afghanistan

US marines from the 2nd Battalion, 8th Marine Regiment of the 2nd Marine Expeditionary Brigade make preparations for helicopter transport at Camp Dwyer Photograph: Manpreet Romana/AFP/Getty Images

The marines were sent in to the region by air and ground transport under cover of darkness Photograph: Manpreet Romana/AFP/Getty Images

The offensive involves nearly 4,000 marines and 650 Afghan forces Photograph: Manpreet Romana/AFP/Getty Images

Operation Khanjar is seen as the first major test of Barack Obama's strategy to wrest the initiative from the Taliban Photograph: Manpreet Romana/AFP/Getty Images

Once on the ground, the marines will meet local leaders and act on their concerns, a spokesman said Photograph: Manpreet Romana/AFP/Getty Images

Marines wait to board helicopters to take part in Operation Khanjar Photograph: Manpreet Romana/AFP/Getty Images

A US captain claimed the marines took many insurgents by surprise Photograph: Manpreet Romana/AFP/Getty Images

The operation was described by officials as the largest and fastest-moving of the war's new phase Photograph: Manpreet Romana/AFP/Getty Images

A night-vision image of marines from 1st Battalion, 5th Marine Regiment, Regimental Combat Team 3 Photograph: Us Marines/EPA

Boarding a UH-60L Black Hawk helicopter at Camp Bastion, in Helmand province Photograph: Us Marines/EPA

Marines from 2nd Marine Expeditionary Brigade, RCT 2nd Battalion 8th Marines Echo Co are airlifted by helicopter into Helmand Photograph: Joe Raedle/Getty Images

Marines run to a new position as they take enemy fire in Main Poshteh Photograph: Joe Raedle/Getty Images

A marine yells to his colleagues as they take enemy fire Photograph: Joe Raedle/Getty Images

Afghan soldiers and US marines on patrol together Photograph: Joe Raedle/Getty Images