The BBC has secured exclusive pictures of British soldiers mounted on Al-Thalab long-range patrol vehicles.
The pictures were taken after the ISIS militants attacked New Syrian Army base of Al Tanaf on the Syria-Iraq border, the news agency reported.
Nine rebels were killed during the attack, as reported by The Telegraph.
The soldiers carrying sniper rifles, heavy machine guns and anti-tank missiles appear to be protecting the rebel base from the jihadist group.
A New Syrian Army spokesman refused to comment on the pictures, but admitted the special forces' involvement in training the rebels.
The official was quoted by the BBC as saying: "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."
The UK Ministry of Defence also declined to comment on the operations being carried out by special forces in Syria.
The New Syrian Army recently failed in an attempt to destroy an important ISIS trading route along the Iraq-Syria border.