In a report submitted to the Organization for the Prohibition of Chemical Weapons' Executive Council, the organization's director-general said the traces were found late last year at Syria's Scientific Studies and Research Centre in Barzah.
Canada's OPCW envoy, Sabine Nolke, said in a speech to the meeting that the discovery, and reports that Syria destroyed equipment and munitions that had been earmarked for further assessment, add to "growing evidence of deliberately false declarations by Syria, destruction of possible evidence, and the alarming likelihood that Syria continues to possess Schedule 1 chemicals."
Schedule 1 chemicals include sarin, VX and sulfur mustard. Director-General Fernando Arias also reported that Syria has refused to issue a visa to the coordinator of an OPCW team that aims to attribute blame for chemical weapons attacks in the country.
Britain's OPCW representative, Ambassador Peter Wilson, called that decision "completely unacceptable" in a speech to the behind-closed-doors meeting. The text of Wilson's speech was published online.
"Syria insists that it has no chemical weapons program — why then is it trying so hard to prevent those who have used chemical weapons on its territory from being identified?" Wilson said. Syria is not a member of the OPCW's 41-nation executive council. The government in Damascus did not immediately respond to a request for comment.
Syria agreed in 2013 to join the OPCW and relinquish its chemical weapons stockpile in a move that averted a U.S. military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta.
Syria's declared chemical weapon stockpile was destroyed in an international operation, but doubts have remained ever since about whether Damascus' declaration was complete, and there have been repeated chemical attacks in the country in the years since Syria joined the OCPW.
The government of President Bashar Assad denies involvement in chemical attacks, but a joint United Nations-OPCW team blamed Damascus for two chlorine attacks and an attack using sarin. The team also accused the Islamic State extremist group of using mustard gas in 2015 and again in September 2016 in Um Hosh, in Aleppo.
Speaking on behalf of the European Union, Finland's representative to the OPCW, Paivi Kaukoranta, also expressed concern at Syria's "continued breach of its obligations" to the organization "due to its failure to provide a complete Declaration and to destroy all of its CW and CW production facilities."