A fact-finding mission by the Organisation for the Prohibition of Chemical Weapons determined that "chlorine was released from cylinders by mechanical impact in the Al Talil neighbourhood of Saraqeb" on February 4, an OPCW statement said.
The team's conclusions were based on finding two cylinders "which were determined as previously containing chlorine."
Environmental samples also "demonstrated the unusual presence of chlorine in the local environment," said the organisation, based in The Hague.
However, in line with its mandate the watchdog did not say which side in Syria's complex seven-year civil war was responsible for using chlorine.
Eleven people had to be treated for breathing difficulties on February 4 after Syrian government raids on the town of Saraqeb, the Britain-based Syrian Observatory for Human Rights said at the time.
Mohammad Ghaleb Tannari, a doctor in a nearby town in Idlib province, also told AFP at the time that his hospital had treated 11 people.
"All the cases we received had symptoms consistent with inhaling the toxic chlorine gas, including exhaustion, difficulty breathing, and coughing," he said.
The OPCW said its team had interviewed witnesses, and found that a "number of patients at medical facilities shortly after the incident showed signs and symptoms consistent with exposure to chlorine."
"I strongly condemn the use of toxic chemicals as weapons by anyone, for any reason, and in any circumstances," said OPCW head Ahmet Üzümcü.
"Such acts contradict the unequivocal prohibition against chemical weapons."
An OPCW fact-finding team is also currently awaiting the results of its difficult mission to the Syrian town of Douma, after medics and rescuers said 40 people died in a chlorine and sarin attack on April 7.
The team exhumed bodies as well as gathering over 100 environmental samples which are being analysed in different OPCW-designated labs.
