British troops in Afghanistan are to hand over responsibility for the Sangin area of Helmand province to US forces, the BBC understands.

BBC获悉，驻阿富汗英军将向美国军队移交桑金地区赫尔曼德省的管辖权。

Sangin has seen some of the fiercest fighting in recent UK military history

Defence Secretary Liam Fox is expected to tell MPs on Wednesday the move could happen by the end of the year.

Almost a third of the 312 UK deaths in the war have happened in Sangin, a volatile¹（不稳定的，爆炸性的） northern district at the heart of the opium²（鸦片，麻醉剂） -growing industry.

Last month Britain handed over command in Helmand to a US general.

Maj Gen Richard Mills, of the US Marine³ Corps⁴, assumed control of all Nato-led International Security Assistance Force (Isaf) troops in Helmand on 1 June.

Difficult questions

Mr Fox is expected to announce Britain will concentrate on Helmand's busy central belt, leaving the north and south to the US.

Of the 312 UK deaths in Afghanistan since 2001, 99 have taken place in Sangin, currently home to 40 Commando Royal Marines.

It is the latest part of the province to be handed over to US control after the town of Musa Qaleh in March and the Kajaki dam last month.

BBC defence correspondent Jonathan Beale said Mr Fox is likely to face some difficult questions, not least from the families of the British servicemen killed in Sangin.

He said: "They may well ask... 'why have we sacrificed so much just to hand over responsibility to the Americans'?".

The Ministry⁵ of Defence will also be concerned it will be seen as a pull out or withdrawal⁶ by the British, with US forces bailing⁷ out（保释，跳伞） UK troops, our correspondent added.

He said Mr Fox, backed by commanders on the ground, is likely to argue the transfer makes military and strategic sense as Sangin is now under US command and there are more US troops on the ground.

'Disingenuous⁸'

He is also expected to tell MPs Britain is not lowering its profile or commitment to Afghanistan.

Sangin has witnessed some of the fiercest fighting the British military has endured since World War II, and contains a mix of rival tribes.

The UK's 8,000 forces in Helmand are greatly outnumbered（超过） by the 20,000 US Marines sent there by President Barack Obama.

Conservative MP and former British army officer, Patrick Mercer, said the handover was a routine move and should under no circumstances be considered a retreat.

He told the BBC: "It's absolutely straightforward⁹（简单的，明确的） and normal in coalition¹⁰（联合，结合） warfare¹¹ like this for units to serve under foreign command, but it's not necessarily desirable.

"There are now enough troops for this no longer to be necessary and any suggestion that British forces are being beaten out of Sangin or returning with their tails between their legs is not just disingenuous（虚伪的，不诚实的） , it's actually disgusting."

The latest British casualty of the war in Afghanistan died in a UK hospital on Monday after being injured in an explosion in the Nahr-e Saraj district of Helmand the day before.

The soldier, from 1st Battalion¹², The Mercian Regiment¹³, was on patrol when he was caught in the blast.