"[Now] the US will take the lead in the north and south, and the UK-led task force Helmand... will concentrate on the centre."

Heavy fighting

Sangin has been the site of heavy fighting for years with almost 100 British troops being killed there, nearly one-third of the 312 to have died throughout Afghanistan.

The announcement came as the Nato military alliance announced the deaths of three foreign troops in the south of Afghanistan.

All three troops, whose nationalities were not given, died in bomb attacks on Tuesday, Nato's International Security Assistance Force (Isaf) said in a statement.

At least 17 Nato and US soldiers have been killed in Afghanistan already this month, after 102 died in June, the deadliest month since the war began in 2001.

The UK established a base in Sangin in the summer of 2006, when several hundred British soldiers were airlifted into the district.

The Afghan army also established a presence in Sangin in April 2007 following a larger Nato operation.

But the deployment has long been controversial in the UK, where some military officials feel Sangin is too small and remote to merit a large army presence.

About 8,000 of the 9,500 British servicemen serving as part of the Isaf force in Afghanistan are based in Helmand province.

British troops have already turned over other mountain valleys in Helmand to the US Marines who arrived in the province last year.