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

'Spread too thin'

The UK established a base in Sangin in the summer of 2006, when a small contingent of British soldiers was airlifted into the district. It was one of several small, remote bases the British army established throughout northern and central Helmand (others were set up in Musa Qala and Now Zad).

But the British army quickly found itself spread too thin as 8,000 troops were scattered across the largest province in Afghanistan. British units killed dozens of Taliban fighters on routine patrols, but they were unable to create any long-term security in Sangin, which became the most dangerous place in Afghanistan for Nato troops.

Almost 100 British troops have been killed there, nearly one-third of the 312 to have died throughout Afghanistan.

Commanders said UK troops had too few soldiers in too many districts of Helmand [AFP]

"The prime minister and I regularly argued in opposition that British troops in Helmand were too thinly spread," Fox said.

Most of Helmand province remains deeply insecure, according to a US defence department report released in April. The report also found that persistent insecurity and a lack of basic services have made the population ambivalent or even hostile towards the government in Kabul.

Anthony King, a professor at the University of Exeter, wrote in the journal International Affairs earlier this year that the British strategy was doomed to failure because it focused on too many districts.

There are "too few operating bases, too thinly spread, with too few troops in them, for the level of hostility the British face" in Helmand, King wrote.

Shift in strategy

The new strategy from Whitehall will see British troops concentrate on securing central Afghanistan. The US will take control of the north - it will assume command in Sangin in November; and it already took over Musa Qala, another northern district, last month.

Stephen Grey, a British journalist who has reported extensively from Helmand, said there was a political dimension to the announcement for David Cameron, the new British prime minister, who wants to demonstrate some changes in the country's Afghan strategy.

"Part of this is making clear that there's a credible strategy in place for this new government," Grey said. "They're trying to prepare the way for what will be an influx of new troops."

"This new strategy will concentrate on the central region... it's also going to make it easier to eventually withdraw."

Fox's announcement comes one day before a conference at London's International Institute for Strategic Studies, where British and Afghan officials will discuss the future of the UK's strategy in Afghanistan.