US President Barack Obama said he was "not optimistic" about Syria's immediate future as the regime and its Russian backers launch a sustained bombardment of civilians in Aleppo.

"I am not optimistic about the short term prospects in Syria," Obama said at a press conference in Peru, warning Syria's second city was likely to fall.

"Once Russia and Iran made a decision to back (Bashar al) Assad in a brutal air campaign... it was very hard to see a way in which even a trained and committed moderate opposition could hold its ground for long periods of time."

Source: AFP