At least 35 people have been reported dead in what witnesses claim was a chemical weapons attack in northern Syria.

Doctors in the rebel-held town of Khan Sheikhoun, in Idlib province, said victims started to choke and faint with foam coming out of their mouths.

Dr Shajul Islam said the hospital where he worked was filling up with patients, posting footage online of patients he said showed symptoms of exposure to sarin.

Rescue workers from the White Helmets group were seen wearing gas masks to protect themselves while treating people lying amid rubble on the ground.

Activists posted video online claiming to show the moment the bombs struck, alleging they were dropped by the Syrian air force as clouds of smoke rose into the air.

Bashar al-Assad's forces have been accused of repeatedly using the nerve agent during the five-year civil war, including in a massacre in Eastern Ghouta that almost sparked military intervention by the UK in 2013.

Damascus has denied the allegations, while Isis is also alleged to have used chemical weapons including chlorine and mustard gas.

Idlib province, in north-western Syria, is a key battleground between regime forces and rebels including Islamist factions linked to al-Qaeda.

It was the destination for rebel fighters evacuated from eastern Aleppo earlier this year, and is widely expected to be Assad's next major target.

Khan Sheikhoun sits near the frontline between regime and opposition held territory, on a key motorway linking the city of Hama with rebel strongholds to the north.