"We understand that Iran has denied that there has been an incident at Fordo. This is consistent with our observations," said Gill Tudor, spokeswoman for the International Atomic Energy Agency.
The reports cited the American news website WND, which said an explosion at the Fordo facility on January 21 had caused major damage and trapped workers.
Iran denied any such blast took place, with a senior lawmaker calling the rumors "Western propaganda" aimed at influencing upcoming talks with world powers on Iran's nuclear program.
Fordo near the holy city of Qom, some 150 kilometers south of Tehran. This site began enriching uranium to purities of 20 percent in late 2011.
Iran is enriching to this level to provide fuel for the Tehran Research Reactor, which produces medical isotopes.
Iran has accused the United States and Israel of being behind the assassinations of nuclear scientists and sabotage attempts, including the Stuxnet computer virus.