Bad weather and shifting battlefronts in Syria's civil war have delayed the delivery of essential supplies to sites where the toxins are being prepared to be sent to Latakia port, the Organisation for the Prohibition of Chemical Weapons (OPCW) said.
"A delay will probably occur," Franz Krawinkler, the OPCW's logistics head told Austrian ORF state television on Saturday.
"Because of various external influences, including the weather... certain logistical supplies that are needed for this transport, could not be delivered in time."
Syria has agreed to abandon its chemical weapons by next June under a deal proposed by Russia and hashed out with the United States, Reuters reported.
Damascus agreed to transport the "most critical" chemicals, including around 20 tons of mustard nerve agent, out of the northern port of Latakia by December 31 to be safely destroyed abroad away from the war zone.
A Russian diplomat was also quoted as saying on Friday that the deadline would be missed because the toxins that can be used to make sarin, VX gas and other agents still faced a potentially hazardous trip to the port of Latakia.