Micah Zenko of the Council on Foreign Relations said the "truly significant decision" could potentially extend well beyond that small force.
US forces were "interspersed among large coherent units of several hundred fighters", he said, explaining that: "You can't give air cover just to individual rebels."
President Barack Obama has shied clear of targeting forces of Bashar al-Assad, the Syrian President. Assad opposes the US intervention but has not actively sought to hinder it. As America co-ordinates policy more closely with Turkey, which is actively backing anti-Assad rebels, that might change. Many Syrians, outside the Kurdish minority whose forces are working closely with the US-led coalition, say they are confused by American policy, which they say is causing more civilian deaths than it is helping their fight against either Isis or the regime.
US-led airstrikes targeting Isis have likely killed at least 459 civilians in the past year, say independent monitors. Airwars, a project aimed at tracking the international airstrikes targeting the extremists, believes 57 specific strikes killed civilians and caused 48 suspected "friendly fire" deaths. It says the strikes have killed more than 15,000 Isis militants.
Meanwhile, two British jihadists high on the CIA's kill list of Isis operatives have been linked to Melbourne man Neil Prakash, regarded as Australia's top recruiter of would-be fighters wanting to join the extremist group.
Ranked third on the list, Junaid Hussain is believed to have played a key role in the online radicalisation of at least one of the men behind the attack on a Prophet Mohammed cartoon contest in Garland, Texas. The only Isis members higher on the kill list are the group's leader Abu Bakr al-Baghdadi and Mohammed Emwazi, the extremist known as Jihadi John. Several US officials, according to CNN, have confirmed there was "intense" interest in taking out Hussain, who operates under the alias Abu Hussain al-Britani, and who has links to Prakash. Another British Isis fighter, Raphael Hostey, who is a close associate of Prakash and has named the Melbourne man as his "co-worker", is also on the list.
- Telegraph Group Ltd, AFP, AP, AAP