Ahrar Al-Sham units and Turkish spec ops backed up by armor are rumored to be on the outskirts of Arima, which is the stepping stone to Manbij.
Rojava is sponsored by the US, and there are even reports of US troops in Manbij itself. But Turkey is of course also a NATO ally.
The day before, a Turkish newspaper claimed that Turkey threatened the US will lose of access to its Incirlik airbase if it doesn’t stop its support for the YPG.
This seems good for the Syrian govenrment. With the SDF occupied, they will not have to compete with them in a race to the Euphrates, and with the SDF refocusing efforts away from Raqqa, the SAA now even stands a chance of getting there first as well.
The Kurds are, obviously, are the biggest losers. The US aren’t going to be doing air support for them, not least because it would put them in direct confrontration with the Turkish Air Force, so Manbij is most likely doomed. Likewise, the line of communication and potential supply route with Afrin canton that Rojava had acquired a week ago when the SAA linked up with it after cutting across Islamic State seems fated to be very short-lived.
That said, this will be a substantial chunk of land that the Turks are carving out for their surrogates. Should the incipient rebel mini-civil war around Idlib result in the victory of Turkish-sponsored Ahrar Al-Sham and its allies over the JFS, then Syrian-held Aleppo could find itself almost entirely surrounded by a rebel force united by being under Turkish support or influence. And Erdogan doesn’t exactly have a reputation for loyalty.