A long overdue concession by the Pentagon to the Russian-Syrian-Iranian camp in an attempt to save its illegal southern Syrian base
CNN reports that Pentagon has told US-trained Syrian rebel groups in southern Syria they must either abandon the fight against the Syrian military, or the joint US-rebel base in Syria's al-Tanf
The Tanf base hosted two rebel groups that we know of. The Maghaweir Al-Thowra (Commandos of the Revolution) and Shuhada al-Qaryatayn (Martyrs of Qaryatayn).
The latter outfit opted for the second option and has exited the US base, and the illegal 55-kilometer kill zone for the Syrian army around it unilaterally proclaimed by Pentagon.
The report is believable and consistent with other news coming from Syria. From April through June the US and Pentagon engaged in a vast escalation in Syria against its government, but since then they've become somewhat more reasonable.
A cease fire for south-western Syria was agreed by Trump and Putin at the G20 meeting in Hamburg, the US has ended CIA's covert aid for rebels, and there are credible reports of Russians and Americans now parleying behind the scenes to avoid a confrontation in the race for the Euphrates valley.
Before anti-Assad rebels were told to leave Tanf if they intend to continue fighting the Syrian army, they were in a position where they could easily strike government forces to the north, then fall back to the safety of the US kill zone around Tanf. This gave the loyalist camp an urgent reason to object to, and possibly act against, the existence of the US kill zone around Tanf, which the US deems necessary for protection of its troops.
In all likelihood the Pentagon dropped the Martyrs of Qaryatayn to maintain working relations with the Russians and forestall any Syrian moves against the base.
On the other hand, the Martyrs can scarcely be blamed for finding the US request to now only fight ISIS ridiculous. Since the Syrian advance to the Iraqi border in early June, the US-rebel base at Tanf has been cut off from ISIS territory. The frontline against ISIS now over 100 miles to the east of Tanf, so there is no way for either the rebels or the US to fight ISIS from there.
Yet the Pentagon has in no way indicated it will be leaving Tanf. To the contrary, since becoming irrelevant to the anti-ISIS fight the US military has instead reinforced the base. Holding onto the illegal base and accompanying occupation zone that are useless for the fight on ISIS shows the US is still very much in anti-Syrian mode, but rebels dragging it into active warfare for southern Syria is a headache it would rather avoid at this point.
UPDATE: Here is public confirmation: