WASHINGTON — The Obama administration’s claim that the Iran nuclear accord provides for airtight verification procedures is coming under challenge from nuclear experts with long experience in monitoring Tehran’s program.

Energy Secretary Ernest J. Moniz has insisted that Iran would not be able to hide traces of any illicit nuclear work before inspectors gain access to a suspicious site. But several experts, including a former high-ranking official at the International Atomic Energy Agency, said a provision that gives Iran up to 24 days to grant access to inspectors might enable it to escape detection.

Olli Heinonen, a former deputy director of the agency, said in an interview that while “it is clear that a facility of sizable scale cannot simply be erased in three weeks’ time without leaving traces,” the more likely risk is that the Iranians would pursue smaller-scale but still important nuclear work, such as manufacturing uranium components for a nuclear weapon.

“A 24-day adjudicated timeline reduces detection probabilities exactly where the system is weakest: detecting undeclared facilities and materials,” he said.