Waves of air strikes pounded rebel-held areas of southern Syria on Thursday as pro-regime forces launched their most intensive bombing campaign yet in a two-week-old offensive.

The renewed assault came after the failure on Wednesday of Russian-brokered talks to end the offensive in Daraa province, which has killed dozens and forced tens of thousands from their homes.

An AFP correspondent on the edge of the rebel-held south of the city of Daraa, the divided provincial capital, said the bombing of rebel neighbourhoods was the most intensive since the launch of the Russian-backed offensive on June 19.

The Syrian Observatory for Human Rights, a Britain-based monitor, said hundreds of missiles and crude barrel bombs were unleashed by Syrian and Russian aircraft overnight.