The BBC has obtained exclusive pictures showing for the first time British special forces operating on the ground in Syria.
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According to eyewitnesses, they were there in a defensive role. But they are carrying an arsenal of equipment including sniper rifles, heavy machine guns and anti-tank missiles.
If IS attacked again they would have been able to put up a considerable fight. A New Syrian Army's spokesman refused to comment on the pictures of the special forces, but acknowledged their help.
He said: "We are receiving special forces training from our British and American partners. We're also getting weapons and equipment from the Pentagon as well as complete air support."
Disruption failed
The Pentagon's programme to train and equip moderate rebels is generally seen as a failure, after many of its fighters were kidnapped or gave their weapons to al-Qaeda in Syria.The New Syrian Army, which draws most of its recruits from Deir Ezzor province, failed in a recent attempt to disrupt a key IS trading route across the Iraq-Syria border, but they have been able to fend off attacks at Al Tanaf.
They were mocked in an IS propaganda video following the failure of the Al-Bukamal attack. IS seized equipment and killed four of its fighters. One captured New Syrian Army recruit was shown being beheaded.
And, embarrassingly for its Western partners, videos of training sessions with western special forces were also included in the IS broadcast.