Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said 11 children were among those killed in day-long strikes by regime and allied Russian aircraft on the last IS-held pocket in Daraa.

The Observatory said that hundreds of air raids and barrel bombs struck several towns controlled by a local IS branch known as Jaish Khaled bin Walid.

The bombing caused severe damage to infrastructure, and "whole neighbourhoods were destroyed", the monitor said.

The Syrian regime and its allies are currently looking to retake full control of Daraa on the border with Jordan and the Israeli-occupied Golan Heights after forcing rebel groups to sue for peace following a bloody offensive.

Syria's conflict has killed more than 350,000 people since it began in 2011 with a brutal government crackdown on protesters.