Iran has reacted sharply to a report from the International Atomic Energy Agency, which says there are signs it may have worked on designing a nuclear weapon.
The United Nations nuclear watchdog stops short of saying explicitly that Iran is developing a nuclear bomb, but its report says aspects of Tehran's nuclear research suggest military aims.
The IAEA says it has information indicating Iran has carried out tests "relevant to the development of a nuclear explosive device", the BBC reports.
The IAEA says the research includes computer models that could only be used to develop a nuclear bomb trigger.
The BBC says Tehran has condemned the findings as politically motivated.
"This report is unbalanced, unprofessional and prepared with political motivation and under political pressure by mostly the United States," said Ali Asghar Soltanieh, Iran's envoy to the IAEA.
It was "a repetition of old claims which were proven baseless by Iran in a precise 117-page response, " he said.
Iran says its nuclear programme is solely to generate civilian power.