Syrian President Bashar al-Assad on Thursday offered an amnesty to armed rebels battling his forces if they surrender, the official SANA news agency reported.

"Everyone carrying arms... and sought by justice... is excluded from full punishment if they hand themselves in and lay down their weapons," SANA said, quoting a presidential decree on the three-month amnesty offer.

The reprieve also includes any rebel who frees a hostage, according to the decree text.

The offer comes as opposition neighbourhoods of Aleppo city are effectively surrounded by pro-government forces, sparking fears for the more than 200,000 people trapped there.

Assad's ally Russia said Thursday it had launched a "large-scale" operation with the Syrian government to open humanitarian corridors for civilians and fighters fleeing the northern city.

Defence Minister Sergei Shoigu told Russian news agencies that three humanitarian corridors were being opened "to aid civilians held hostage by terrorists and for fighters wishing to lay down their arms" and one more corridor to the north of the city for rebels to flee with their weapons.

Assad has issued several amnesties in recent years, including one in July 2015 for people who have dodged service or defected from the army.

Syria's UN envoy Staffan de Mistura said this week he hopes peace talks aimed at ending more than five years of brutal conflict could resume at the end of August.

More than 280,000 people have been killed in Syria since the war began in March 2011 with anti-government protests that were met with a brutal regime crackdown.