UNITED NATIONS — The last of the chemicals to be shipped out of Syria contain the ingredients needed to make sarin, the lethal nerve agent used in the attack on a Damascus suburb last August, the United Nations official who is coordinating the arsenal’s destruction said on Thursday.

The material is stored in an airfield controlled by President Bashar al-Assad’s military forces not far from Damascus, and was transferred there from another site about 19 miles away that has since been overrun by insurgents in the civil war, the United Nations official, Sigrid Kaag, said in an interview.

“It is safe and secure,” she said, adding that the approximately 100 tons of chemical substances at the site go into making the nerve agent. “You’ve got the main ingredients, the precursors to produce sarin.”

Ms. Kaag said the last batch of the arsenal, which represents about 8 percent of the total declared by the Syrians, cannot be safely extracted yet because the roads to the location have not been secured by government forces. “There is a lot of fighting taking place,” she said. “It’s not a situation where you would want a chemical weapons convoy passing through.”