With the Turkish-led rebels already taking over the city of Manbij and moving toward Kobani on the Turkish border, critics of the Turkish-backed operation against Kurdish-led troops warn that it might further destabilize Syria.
Videos of US armored vehicles entering Kobani were shared via social media on Saturday, according to Kurdish stations.
A four-day truce was established upon Thursday, but the SDF warned of an impending rebel onslaught against the city.
Iran, a key supporter of the former government, has suffered a serious setback as the rebel control prevents it from providing routes via Syria to Hezbollah, its proxy in Lebanon.
Hezbollah commander Naim Qassem said on Saturday that the land corridor that Assad’s administration had established for the shipment of weapons and supplies from Iran to Lebanon was no longer there.