Turkish-backed rebels in Syria are advancing on Dabiq, a symbolically important 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 comes as US and Russian envoys meet in Switzerland to discuss possible routes to a new ceasefire.
But diplomats have played down hopes of any breakthrough at the talks.
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.
Turkish President Tayyip Erdogan confirmed fighters were moving on Dabiq, which lies about 10km (6 miles) from the Turkish border.
Rebel fighters backed by Turkish airstrikes have been edging closer to the town for days, seizing villages around it and all but isolating it.
A bombardment was taking place as part of the offensive on Saturday, a monitoring group and a rebel commander said.
Dabiq is important to IS because it is named in Islamic apocalyptic prophecies as the site of an end-of-times showdown between Muslims and their enemies. The militant group named its magazine after the town.
The advance on Dabiq is part of a wider offensive launched by an alliance of Syrian rebel groups, supported by Turkish forces, in late August.
They are trying to drive IS militants and Syrian Kurdish fighters from an area along Turkey’s border with Syria. Since it began, they have captured the key towns of Jarablus and al-Rai.