It was a long, painful, difficult and disappointing meeting, said the UN special envoy for Syria

Syria announced a new offensive against rebel-held areas of Aleppo on Thursday while diplomats failed to find a way in New York to revive a US and Russian-brokered ceasefire that collapsed this week.

Warplanes mounted the heaviest air strikes in months against rebel-held districts of Syria's commercial hub and largest city, dealing a fresh blow to efforts to end Syrian civil war that has raged since 2011.