The International Atomic Energy Agency says its team visiting Iran has been denied access to a military site where it suspects suspicious nuclear activities are being carried out.
IAEA director-general Yukiya Amano says no deal has been reached on inspecting Parchin, south of Tehran, despite "intensive efforts". Two deals of talks were no successful.
An earlier round of discussions in January also failed to produce a result.
IAEA inspectors had sought to clarify the "possible military dimensions" of Iran's nuclear programme.
Iran insists it is purely for peaceful purposes, but the West suspects it is seeking to build a nuclear weapon.
In November, an IAEA report indicated that Iran built a large explosives containment vessel at Parchin in 2000 to conduct hydrodynamic experiments.
Iran's ambassador to the IAEA, Ali Asghar Soltanieh, said at the time that there were no nuclear-related activities at Parchin.