At least 18 people have been killed and dozens wounded in a suspected chemical attack on a rebel-held town in north-western Syria, activists say.

The Syrian Observatory for Human Rights reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.

The opposition Local Co-ordination Committees network posted photos of those it said had died of asphyxiation.

The Syrian government strongly denies the accusations.

Chemical weapons have also been found in the hands of Islamic State militants and more specifically the blister agent sulphur mustard.