Washington finally admits 'US-backed Syria rebels' can only fight IS, not Assad regime

US-backed opposition forces in southern Syria are only authorised to fight the Islamic State group and not Bashar al-Assad's regime, Washington has confirmed.

It follows consistent reports that Washington has forced rebel groups in the Syrian Desert region to pledge they will only fight IS in return for US arms and training.

On Wednesday, a senior US officer appeared to confirm Washington's priority in Syria is not the removal of Assad but the defeat of the Islamic State group.

"The coalition supports only those forces committed to fighting [IS]," the US-led anti-IS coalition spokesman Col. Ryan Dillon told CNN.

The recent decision has prompted one rebel group to leave the US-run al-Tanf base in southern Syria, close to the Iraqi and Jordanian borders, when they were offered the choice of US arms or fighting Assad.

"The Shohada al-Quartyan [rebel group] have made it known that they may want to pursue other objectives," Dillon added. 

"The coalition is making it clear to Shohada al-Quartyan leadership that if they choose to pursue other objectives, the coalition will no longer support their operations."

Anti-IS forces in southern Syria are mostly part of a Jordan-based, US-backed train-and-equip programme that vets fighters to ensure they won't use their weapons against the Assad regime.

They operate from US-supplied bases in southern Syrian Desert including al-Tanf, which has been a main hub for the battle against IS in the south.

It is surrounded by a 55km wide "de-conflict zone" that surrounds al-Tanf, which is intended to prevent pro-regime forces clashing with opposition fighters in the area.

Several times Iranian-backed, pro-Damascus forces have been bombed by US aircraft when it appeared they were heading for al-Tanf.

Shohada al-Quartyan forces recently left this protective zone to embark on a new campaign against the Syrian regime, Tillon told CNN.

Another US official confirmed to the broadcaster that Washington was primarily focused on defeating IS, not the downfall of Assad even if the al-Tanf base is surrounded by pro-Damascus forces. 

"We are not in the business of fighting the regime," an official told CNN.

"They can't have multiple objectives and we need to be singularly focused on fighting IS."