A small contingent of US special forces will be sent as part of an “advise and assist” mission to the Syrian rebels fighting against Islamic State militants, Reuters reported citing unnamed US officials.

President Barack Obama is expected to make a full announcement Friday.

Unnamed US officials told Reuters that 20-30 special forces operatives will be sent to Syria as military advisers, presumably to the US-backed rebel groups.

Obama has previously refused to send US troops to Syria, saying that local Sunni Arabs would have to fight IS (also known as ISIS and ISIL) on the ground, while the US-led coalition supported them from the air.

US special operatives are part of a 4,500-strong force already deployed in Iraq, advising and assisting the Iraqi army and militias in their fight against Islamic State forces. Last week, one of the US advisers –Sergeant Joshua Wheeler of the secretive Delta Force – was killed during a hostage rescue raid in northern Iraq.

Russia has been conducting precision airstrikes on IS positions in Syria at the request of President Bashar Assad since September 30, which according to the Russian General Staff have already resulted in a large-scale militant retreat and the loss of much of their weaponry and equipment.

However, operations without a nod from the official government or UN authorization don’t have legitimacy under international law.

“Any operations – air based operations, ground based operations – in Syria by American forces will be illegal,” Konstantin Kosachev, the head of Russia’s parliamentary committee on foreign affairs, told RT earlier this week, commenting on the potential involvement of US ground troops against ISIS in Iraq and Syria.

Obama’s announcement comes as diplomats from 20 countries gather in Vienna for talks on the political solution to the Syrian civil war.