Nowhere to run: ISIS are 'completely besieged' in its last major Syrian stronghold of Aleppo as Assad's forces and rebel groups attack from all sides 

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The Islamic State group is '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.

Syria's army and its allies advanced towards the northern Islamic-State held city on Monday, cutting off the last main supply route that connects to militant strongholds further east towards Iraq.

Islamic State militants are now effectively besieged in the area, by the army from the south and by Turkish-backed rebels from the north, as Damascus and Ankara race to capture the largest IS stronghold in Aleppo province.

Fighters from the Free Syrian Army disembark from a vehicle near the town of Bizaah northeast of the city of Al-Bab on February 4, 2017

Two fighters from the Free Syrian Army stand next to an armoured pickup truck, one carrying a machine gun, near the town of Bizaah northeast of the city of Al-Bab

'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 said, referring to the jihadists' de facto capital in Syria.

The Syrian Observatory for Human Rights, a British-based group monitoring the war, said the army and the Lebanese Hezbollah group made gains southeast of al-Bab overnight.

Backed by air strikes, government forces and their allies severed the main road that links the city near the Turkish border to other ISIS-held territory in Raqqa and Deir al-Zor provinces.

Northern Syria is one of the most complicated battlefields of the multi-sided Syrian war, with Islamic State now being fought there by the Syrian army, Turkey and its rebel allies, and an alliance of U.S.-backed Syrian militias.

The Syrian army's advance towards al-Bab risks triggering a confrontation with the Turkish military and its allies, groups fighting under the Free Syria Army banner, which have been waging their own campaign to take the city. 

Fighters from the Free Syrian Army sit inside an armoured vehicle near the town of Bizaah

A man carries a wooden door to be used for heating in Bab Antakia, in the Old City of Aleppo, Syria

Turkish troops and FSA rebels clashed heavily with ISIS militants around the town of Bazaa, east of al-Bab, in recent days, the Observatory said. 

Turkish-backed forces had briefly captured the town before Islamic State suicide bombers pushed them out on Saturday.

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 ISIS since fully recapturing Aleppo city in December, in the biggest blow to rebels who have been fighting to topple his regime since 2011.

Free Syrian Army fighters sit inside an armoured vehicle north-east of the city of al-Bab

A fighter from the Free Syrian Army sits in the back of an armoured pickup truck while another one stands opposite to him as he mans a mounted machine gun

ISIS 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 ISIS 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.