The Islamic State has told opposition fighters in the Aleppo if they don't make way for its fighters to combat regime forces in a defacto truce then civilian deaths will be their responsibility.

The request from IS (Isis/Daesh) to fight the Syrian government forces in Aleppo comes as Syrian President Bashar al-Assad unleashes an unprecedented onslaught in the country's most populous city with the backing of Russian air power.

"Make way for us to fight the regime army. If you fight us the blood is on your heads," IS told the opposition fighters, in a message translated by Memri, the jihad and terror threat monitor.

The message came as IS' propaganda outlets reported the defection of opposition fighters from Aleppo to its lines north of the city. It claimed in a prearranged deal the fighters handed themselves over at an IS checkpoint where their safety was guaranteed.

Isis in Raqqa
Members loyal to the Islamic State wave flags as they drive around Raqqa Reuters

As supply lines to Aleppo are squeezed, 300,000 people still living in the city are at risk of being trapped. Siege tactics have already been used in Syria's ground war.

Tens of thousands have fled Aleppo to the Turkish border since the start of renewed government attacks. Turkish authorities have warned as many as 600,000 refugees could flood over the border if Russian and regime air strikes continue.

Advances by Assad forces and Kurdish militias on Aleppo and its surroundings have increased pressure on IS in its Raqqa stronghold. For the first time since 2014, the Syrian army could advance into the IS-held city.

Reuters reported that the army had captured positions at the provincial border between Hama and Raqqa and intends to advance further.