US forces were engaged in a fierce fight in Helmand as thousands of Marines forged ahead with Operation Khanjar, their offensive to drive Taleban fighters out of the southern part of the Afghan province.

British forces in the central areas of Helmand, meanwhile, announced the seizure of 13 strategically important canal crossing points.

In the largest offensive operation by US Marines since the retaking of Fallujah in Iraq in 2004, Brigadier-General Larry Nicholson said that the American troops had run into stiff resistance in some areas. He confirmed that the Marines had taken casualties, with the death of a first soldier in combat.

The US push into the southern districts of Nawa and Garmsir is intended to clear an area known to British forces as “The Fishhook” because of the shape of the Helmand River at that point. About 4,000 Marines are involved in the operation over a 55-mile front. The Taleban has controlled the area for the past three years.

Brigadier Nicholson said that while the 1st Battalion of the 5th Marine force was making steady progress elements of the 2nd Battalion of the 8th Marines had run into fierce opposition. “For 2/8 there is a hell of a fight going on in the southern quarter of the sector,” he told reporters.

British forces seized 13 canal crossing points in the central district of Nad-e-Ali, in what was described as one of their “most strategically significant operations”. The seizure of the Shamalan Canal, as part of Operation Panchai Palang (Panther’s Claw), drives a wedge into Taleban-held territory in Nad-e-Ali.

British commanders believe that they will be able to control the movement of supplies and Taleban fighters into the heavily populated districts west of the provincial capital, Lashka Gar, before the presidential election in August. New checkpoints occupied by the Welsh Guards, Light Dragoons and 2nd Royal Tank Regiment have been under repeated attack since the canal was taken.

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