Russian and Syrian forces intensified airstrikes and shelling in rebel-held northwestern Syria overnight, the heaviest assault since the area was declared a demilitarized zone under a Russian-Turkish deal, residents and medics said on Thursday.

The targeted villages and towns in northern Hama and southern Idlib fall within a buffer zone agreed last September between Russia and Turkey as part of a deal which averted a major offensive on the area.

Schools, health facilities and residential areas have been hit, United Nations regional humanitarian coordinator Panos Moumtzis told Reuters on Thursday. ¡°The barrel bombing is the worst we have seen for at least 15 months.¡±