The head of the UN nuclear agency, the IAEA, has proposed sending a high-level mission to Iran, to address new fears about a possible military dimension to the country's nuclear programme.
IAEA Director-General Yukiya Amano said there was credible information Iran had carried out activities relevant to the development of a bomb.
He said there might be undeclared nuclear material and activities.
Tehran says its programme is for peaceful purposes.
"Our technical experts have spent years painstakingly and objectively analysing a huge quantity of information from a wide variety of independent sources, including from a number of member states, from the agency's own efforts and from information provided by Iran itself. The agency finds the information to be, overall, credible," Mr Amano said in a statement.
"It is consistent in terms of technical content, individuals and organisations involved, and timeframes. The information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."
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He hoped a date for the visit would be agreed soon, he added.