GENEVA — After weeks of deepening foreign frustration over missed deadlines and other delays in Syria’s elimination of its chemical weapons, the international mission policing the process reported on Wednesday that Syria had delivered a significant consignment of mustard gas, one of the deadliest toxic agents, to the Syrian port of Latakia to be exported and destroyed.

Sigrid Kaag, the coordinator of the international mission of the United Nations and the Organization for the Prohibition of Chemical Weapons, called the delivery “an important step” toward eliminating Syria’s chemical weapons in the middle of the three-year-old civil war there.

The latest shipment came after a series of meetings at the Organization for the Prohibition of Chemical Weapons in recent days, aimed at resolving a rift between Syria and its allies, notably Russia, Iran and China, and most Western governments over how to deal with the failure of President Bashar al-Assad’s government to honor deadlines for disposing of the arsenal, which came to about 1,200 tons of deadly chemical agents and materials used to compose those weapons.

Ahmet Uzumcu, director general of the organization based in The Hague, welcomed the latest shipment as “encouraging,” but added, “Much work nonetheless remains to be done, and we look to the Syrian government to accelerate its efforts to transfer the remaining chemicals in regular, predictable and systematic movements.”