The largest rebel group in Syria who is fighting ISIS, the Islamic Front, is led by admitted al-Qaeda operative Abu Khaled al-Suri (who leads Ahrar al-Sham), also fighting ISIS is al-Qaeda’s official Syrian branch, al-Nusra Front.

Via Telegraph:

The United States and Gulf countries have been secretly backing efforts by opposition rebels to destroy al-Qaeda’s most extreme wing in Syria, diplomats and rebels involved in the plan have told The Telegraph.

As Western leaders publicly push the Syrian regime and the opposition to the Geneva II peace conference that begins Wednesday Washington has also been quietly supporting moves by Saudi Arabia and Qatar to give weapons and cash to rebel groups to fight al-Qaeda’s Islamic State of Iraq and al-Shams (ISIS) in Syria.

One source said the US was itself handing out millions of dollars to rebel groups best equipped to take on the extremists while another confirmed America was providing non-lethal aid.
The development marks a new phase in the conflict, with international backers working directly with rebel commanders to target al-Qaeda cells, who are seen as a major threat by Western intelligence agencies.

“Everyone is offering us funding to fight them,” said one commander in a rebel group affiliated to the Western-backed Supreme Military Council. “We used to have no weapons with which to fight the regime, but now the stocks are full.” […]

A resident living close to bases for the Army of Islam and the Syrian Revolutionary Front in Syria’s Idlib province said he had seen 15 trucks “filled with weapons going to the bases”.

Washington did not directly give arms, he said, but backed Saudi Arabia in its funding of the groups. The United States has, however, also been giving $2 million in cash every month as an unofficial hand out, splitting that amount between western friendly rebel groups, the source added.