Barrel bombs dropped by President Assad’s air force have caused extensive damage to Palmyra’s medieval citadel and forced most of the city’s few remaining residents to flee deeper into Islamic State territory.

At least 13 of the bombs — oil drums packed with explosives and shrapnel — struck the citadel during three days of heavy bombardment; part of an intensified aerial campaign by the Syrian regime against Isis since taking delivery of new Russian weapons and warplanes.

Khaled al-Homsi, an activist from Palmyra, said that almost a third of the walls of the 13th-century Mamluk citadel, a Unesco world heritage site, had been badly damaged in the latest bombardment.