Turkish-backed Syrian rebels are advancing on Dabiq, a symbolic stronghold of so-called Islamic State.
The small town holds great value to IS because of a prophecy of an apocalyptic battle, and features heavily in its propaganda.
The operation began as US and Russian envoys met in Switzerland to discuss possible routes to a new ceasefire.
No statement was issued after the talks although both the US and Russian envoys spoke of “ideas” emerging.
Since a brief truce collapsed last month, Syrian government forces backed by Russian air strikes have intensified their bombardment of rebel-held areas in Aleppo.
Aid agencies say a 72-hour ceasefire is urgently needed to allow supplies in and civilians out of devastated areas in the east of the city.