There was a time, early in the Syrian War, when the US was heavily backing large numbers of rebel groups. Most of that aid has been frozen, and those rebels largely defeated. Yet those rebels aren’t gone, they’re just looking for new paymasters.
And they’ve been finding them. Lebanese Shi’ite militia Hezbollah has been hiring former US rebels by the thousands. Told the US was not going to pay them anymore, they were advised to see what the Syrian government’s allies were offering. At $250 a month from Hezbollah, they’re paying better than the Syrian Army.
Not all the rebels were keen to abandon the fight and take a salary from a Syrian government or Iranian-align militia, however. For them, the recruitment is in the north, where the Turkish government has been recruiting heavily.
Even if Turkey is nominally a US ally, that doesn’t mean these rebels are on America’s side, however. Those rebels are being used chiefly to attack the US-backed Kurdish YPG in the country’s northeast. Perhaps even more likely than the new Hezbollah hires, Turkey’s militias are more likely to get in immediate, direct conflict with the US.