The BBC quoted the Syrian Observatory for Human Rights reporting that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
It said the opposition Local Co-ordination Committees network also posted photos of those it said had died of asphyxiation.
The Syrian government has repeatedly denied using chemical weapons in the battle against rebels.
But investigations last October by the United Nations and the Organisation for the Prohibition of Chemical Weapons blamed the government forces for the use of chlorine as a weapon at least three times between 2014 and 2015.
The investigations also found that Islamic State militants had used sulphur mustard blister agent as a weapon.
