Their symptoms included dilated pupils, muscle spasms, and involuntary defecation, which are consistent with sarin or similar agents, Medecins Sans Frontieres said.
The US believes sarin gas was dropped by Syrian aircraft.
MSF said its medical teams also reported smelling bleach at some hospitals treating victims, suggesting they were also exposed to chlorine gas.
More than 70 people were killed in the rebel-held town of Khan Sheikhoun this week.
The World Health Organisation also claimed the victims seemed to show symptoms associated with exposure to a nerve agent, saying they suffered “acute respiratory distress” and had an “apparent lack of external injuries”.
What is sarin gas?
Sarin is a clear, colourless and tasteless liquid that has no odour in its pure form.
It is made up of four common chemical compounds: dimethyl methylphosphonate, phosphorus trichloride, sodium fluoride and alcohol.
Sarin is man-made and part of a class of chemical weapons called G-series nerve agents developed by German scientists during World War Two. Other agents include tabun, soman and cyclosarin.
At room temperature, G-series nerve agents are volatile liquids, with sarin being the worst.
How does sarin gas kill?
Sarin interferes with the neurotransmitter acetylcholine, which acts as an off switch for glands and muscles. Blocking that switch leads to over-stimulation of muscles.
The extent of poisoning depends on how much chemical a person is exposed to and for how long.
Large doses of sarin can cause loss of consciousness, convulsions, paralysis and respiratory failure and death.
Because sarin evaporates quickly, it presents an immediate but short-lived threat.
What is chlorine gas?
Chlorine in liquid or tablet form is found in some household products and often used to disinfect swimming pools. But it is sometimes in the form of a poisonous gas, which is a greenish-yellow colour.
It is very reactive with the human body and very toxic.
It was used as an agent of warfare by Germany during the First World War.
Chlorine remains the simplest chemical weapon and reappeared on the battlefield during the Iraq War in 2007 and allegedly in Syria.
How does chlorine gas kill?
Chlorine gas can react with water in the lining of lungs and form hydrochloric acid.
This destroys the lungs’ ability to take in oxygen. People can die from lack of oxygen when enough gas is used.
So how is chlorine safely used in swimming pools?
The quantity of water in a pool is so large that the chlorine is diluted to the point where it is only very mildly toxic.
If people swim a lot with your eyes open, their vision can become blurry for a few hours afterwards.