They have been forced to take on a more active role rather than just training the rebel forces due to a sustained attack near the Jordan-Syria border, The Times reported.
"British special forces based in Jordan frequently cross into Syria to assist the New Syrian Army (NSA), which has been holding out in the south-eastern village of al-Tanf.
"The rebel unit, made up of former Syrian special forces who defected from President Assad's army, was retrained by the British and Americans," the newspaper said.
Almost two years ago, Prime Minister David Cameron was defeated in the House of Commons when he failed to secure enough votes from MPs for airstrikes on Syria against the Assad regime.
However, since then British and American special forces have been deployed to train and assist rebel groups to destroy ISIS, which is now fighting on four fronts in Syria and Iraq.
The UK ministry of defence has refused to comment on the role of the country's special forces, either in Syria or Libya.