The Syrian Observatory for Human Rights reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating some of the survivors, medics and opposition activists said. The Syrian government has repeatedly denied using chemical weapons.
If confirmed, it would be one of the deadliest chemical attacks in Syria since the civil war began six years ago.
Mohammed Rasoul, who is in charge of a charity ambulance service in Idlib, told BBC Arabic that he received news of the air strikes at about 06:45 (03:45 GMT).
When three of his ambulances arrived at the scene about 20 minutes later, medics found people choking in the street, he said.
“Our team is still there, moving patients from one place to another because of overcrowded hospitals.”
“I am speaking to my team and they are doing fine, but the situation over there is very bad and most of those who are suffering are children,” he added.
Mr Rasoul reported that 67 people had died and that 300 were injured, while the pro-opposition Step news agency put the death toll at 100.
The Syrian Observatory cited medical sources in Khan Sheikhoun as reporting that 11 children were among the dead, and that the symptoms among those affected included fainting, vomiting and foaming at the mouth.
The pro-opposition Edlib Media Centre (EMC) posted a large number of photographs of people receiving treatment, as well as images showing what appeared to be the bodies of at least seven children in the back of a pick-up truck. It was not immediately possible to verify the images’ authenticity.
The Syrian Observatory said it was unable to confirm the nature of the substance that was dropped, but the EMC said it was thought to be the nerve agent sarin.