The United States expressed concern on Thursday that Syria’s government might be harboring undeclared chemical weapons, hidden from the internationally led operation to purge them over the past year, and that Islamist militant extremists now ensconced in that country could possibly seize control of them.

The assertions by Samantha Power, the United States ambassador to the United Nations and current president of the Security Council, were made after the Council received a private briefing on the Syria chemical weapons disarmament effort from Sigrid Kaag, the United Nations official appointed last year to coordinate it. Under Ms. Kaag, 96 percent of Syria’s declared chemical weapons stockpile, including all of the most lethal materials, have been destroyed.

But Ms. Kaag told reporters after the briefing that Syria had yet to address what she described as “some discrepancies or questions” about whether it had accounted for all of the chemical weapons in its arsenal. She also said Syria had yet to destroy seven hangars and five tunnels used for mixing and storing the weapons — which is required under the chemical weapons treaty that Syria has signed. The Organization for the Prohibition of Chemical Weapons, the Hague-based group that collaborated with the United Nations in overseeing the Syrian chemical disarmament, is now responsible for ensuring that Syria honors its promise.

“It’s a discussion that’s continuing in Damascus as well as The Hague,” Ms. Kaag said.

She sounded more diplomatic than Ms. Power, who suggested in her remarks to reporters later that the Syrian government’s history regarding chemical weapons use raised skepticism about its commitment. She noted that Syria had repeatedly violated a timetable for the export and destruction of the chemicals, delaying the operation well past the June 30 target date.