The Syrian government has lost control of its second city, Aleppo, for the first time since the country’s civil war began.

Led by Islamist militant group Hayat Tahrir al-Sham (HTS), opposition forces have seized the city and are pushing south, observers told news agency AFP.

In a statement, President Bashar al-Assad vowed to "defend [Syria's] stability and territorial integrity in the face of all terrorists and their backers".

Russia, an ally of Assad, has carried out "a series of air strikes" in Syria early on Sunday.