The UN envoy to Syria has warned of a “perfect storm” in north-west Syria if the government goes ahead with its threatened offensive against rebels.

Staffan de Mistura called for humanitarian corridors to be set up to allow civilians to be evacuated temporarily.

With rebels defeated in most of Syria, such an offensive could prove to be the last big battle of the civil war.

Russia, the government’s main military ally, has also stepped up its rhetoric.

Foreign Minister Sergei Lavrov said “terrorists” must be wiped out in Idlib, accusing them of using civilians as human shields.

Both he and his Syrian counterpart, Walid Muallem, accused rebels of preparing to stage a chemical attack in Idlib in order to blame pro-government forces and draw new US military retaliation.

Mr de Mistura said both the government and the rebels had the ability to make chlorine-based chemical weapons.