The Islamic State group was "completely besieged" in Al-Bab after forces loyal to Syria's regime cut off a road into the jihadists' last major stronghold in Aleppo province, a monitor said Monday.
"Al-Bab is now completely besieged by the regime from the south, and the Turkish forces and rebels from the east, north and west," said the Syrian Observatory for Human Rights.
It came after "the regime's forces and allied militia seized the only and last main road used by the jihadists between Al-Bab and Raqa," Observatory chief Rami Abdel Rahman told AFP, referring to the jihadists' de facto capital in Syria.
Regime forces were backed by fighters from Lebanese Shiite movement Hezbollah and by Russian artillery, said the Britain-based Observatory, which relies on a network of sources on the ground for its reports.
The town of Al-Bab, 25 kilometres (15 miles) south of the border with Turkey, is seen as a prize by nearly all sides in the complex war.
The regime of Syrian President Bashar al-Assad has refocused on IS since fully recapturing Aleppo city in December, in the biggest blow to rebels who have been fighting to topple his regime since 2011.
IS is among several jihadist movements that have shot to prominence during the conflict, which has left more than 310,000 people dead and has forced millions more from their homes.
Assad's forces were also locked in fighting with IS in the central province of Homs at the weekend, the Observatory said.
It reported that the troops had captured the Hayyan oilfield west of the celebrated desert city of Palmyra.
They also fought back against IS around Al-Seen military airport northeast of Damascus, said the monitoring group.