Turkey says it has "concrete evidence" the nerve agent Sarin was used in a suspected attack by Syrian government forces a week ago, state media report.
Health Minister Recep Akdag was quoted by the Anadolu news agency as saying traces of one of the decomposition products of Sarin had been detected in the blood and urine of several victims.
At least 89 people were killed in the incident in rebel-held Khan Sheikhoun.
Syria's government has repeatedly denied using any chemical weapons.
The US has rejected the assertion, however, and on Friday it carried out a missile strike on the Shayrat airbase, where it said a Syrian Air Force jet that attacked Khan Sheikhoun with chemical weapons took off.
President Bashar al-Assad's ally Russia, which says the Syrian jet had in fact hit a rebel depot full of chemical munitions, denounced the "illegal action" by the US.
A number of casualties were treated at hospitals in Turkey, which opposes Mr Assad, following last Tuesday's incident in neighbouring Idlib province.
The World Health Organization said their apparent lack of external injuries and similar symptoms, including acute respiratory distress, suggested they had been exposed to organophosphorus chemicals, a category that includes nerve agents.
On Tuesday, Mr Akdag announced that a chemical known as isopropyl methyphosphonic acid (IMPA) had been "identified in the blood and urine tests conducted on samples taken from the victims".
Once exposed to air and water, Sarin breaks down into IMPA and methylphosphonic acid (MPA).
Earlier on Thursday, Russia's president urged the Organisation for the Prohibition of Chemical Weapons (OPCW) to open a full investigation into last week's incident.
Vladimir Putin also claimed rebels were planning "acts of provocation" using chemical weapons to encourage further US strikes on government forces.
A Russian military spokesman said that the Syrian government was prepared to allow OPCW experts to visit the Shayrat airbase to search for chemical weapons.