GENEVA — Syria is likely to miss its year-end deadline for getting its most deadly chemical weapons out of the country despite an international effort to mobilize the resources needed to do so, according to the United Nations and the international chemical weapons monitoring group overseeing the program.

Syria has until mid-2014 to destroy its chemical weapons program under the deal struck by Russia and the United States in September. To meet that challenging timetable, it agreed with the Organization for the Prohibition of Chemical Weapons to remove some 500 tons of its most toxic chemicals by the end of this year and the remaining roughly 700 tons of chemicals in its stockpile by early February.

"At this stage, transportation of the most critical chemical material before 31 December is unlikely," the United Nations and the chemical weapons group said in a joint statement released on Saturday. They said that volatile security in Syria had "constrained planned movements" and that logistical problems and bad weather had contributed to the delay.

The O.P.C.W. had warned of possible delays when it approved the plan, and the statement noted the "important progress" Syria has made in dismantling its chemical weapons program in the past three months.

The plan the group agreed to earlier this month called for Syria to transport the "critical" chemicals, including some 20 tons of sulfur mustard and precursors for making sarin and VX nerve gas, from 12 storage sites to the port of Latakia. Danish and Norwegian ships are to l then transport them under naval escort to an Italian port for transfer to an American vessel fitted with special equipment for destroying them at sea.

Once movement of the chemicals gets underway the mission can be conducted quite quickly, but it appears that Syria has not yet started transporting any chemicals, according to observers familiar with the mission, who spoke only on the condition they not be identified publicly because of the sensitivity of the issue.

Syria now has "virtually all" of the logistical and security assets it needs to undertake the movement of its chemical weapons, Ahmet Uzumcu, head of the O.P.C.W. said in a statement released after a meeting Friday in Moscow of all countries providing maritime support for the operation. Russia, which has shipped armored vehicles to Syria to transport the chemicals, is due to provide security at the Latakia port and, with China, Denmark and Norway, has offered to provide naval escorts for part of the voyage.

But transporting the chemicals by road to Latakia poses a particular challenge. Syrian government forces, which reportedly control the road from Damascus to the port, may still face the danger of rebel attacks.

Mr. Uzumcu is to report on Syria's progress to a meeting of the monitoring group's executive council at The Hague on Jan. 8.