Despite Thursday’s announced cease-fire, Turkey’s week-old incursion into northeast Syria is already proving to be a propaganda windfall for the extremist group, which in recent months had been making faltering attempts at a comeback in parts of eastern Syria controlled by the U.S.-backed Syrian Democratic Forces, the analysts said.

The Islamic State’s official ­media arm taunted the SDF on Thursday, calling it an abandoned American ally and warning that further attacks were coming.

“The withdrawal of the U.S. from northern Syria and the destabilization that has ensued has created a perfect situation for ISIS to capitalize on,” said Rita Katz, executive director of the SITE Intelligence Group, a private firm that tracks online extremist activity.

An editorial in the Islamic State’s al-Naba newsletter said the Kurds were only the latest of Washington’s allies to be dumped after they were no longer needed.