Britain said on Tuesday it had sent military personnel to the southern Afghanistan province of Helmand following reports that the district capital Sangin was on the verge of falling to Taliban forces.
Helmand's governor said on Monday that Afghan police were holding out against Taliban fighters who had surrounded their compound and the district governor's building in Sangin but roads into the town were completely controlled by the militants.
He warned the situation risked slipping entirely out of control. At least 90 Afghan security forces have been killed since Friday.
Britain, which ended combat operations in Afghanistan last year but has about 450 troops there to mentor and support the Afghan army and security forces, said a small number of personnel had been deployed to Helmand in an advisory role.
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The Times newspaper reported that a unit of about 30 soldiers from Britain's elite Special Air Service (SAS) and up to 60 US special forces had been sent to bolster the Afghan forces defending the town, but the Ministry of Defense (MoD) said the team would not be engaged in fighting.
"These personnel are part of a larger NATO team, which is providing advice to the Afghan National Army," an MoD spokeswoman said. "They are not deployed in a combat role and will not deploy outside the camp."
Helmand, a major center of opium cultivation and a traditional Taliban heartland, has been the scene of fierce fighting for months as the insurgents ramp up attacks.
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British and American forces struggled for years to control the volatile province, and many of the more than 450 British servicemen and women killed in Afghanistan lost their lives fighting there.
Britain's heaviest losses were sustained in Sangin, meaning the town holds symbolic significance for the troops.
Richard Streatfield, a major with the British army who spent seven months in Sangin in 2009 and 2010, told the BBC it was "hugely disappointing" to see the town under siege by the Taliban again. "I won't deny, on a personal level, it does make you wonder — was it worth it?" he said.
"Because if the people we were trying to free Afghanistan from are now able to just take it back within two years, that shows that something went badly wrong at the operational and strategic level."
British combat operations were ended in October 2014, with Prime Minister David Cameron saying that "there is no prospect of the UK going back to fight in Afghanistan because there is now an effective army and police force in place, a process that had taken "hard, patient work."
But the effectiveness of those security forces has since been seriously called into question. Last week, the Pentagon warned of deteriorating security in Afghanistan and assessed the performance of Afghan security forces as "uneven and mixed."
Six American troops were killed in Afghanistan on Monday when a suicide bomber on a motorbike struck their patrol near Bagram air base.
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