A report by the International Atomic Energy Agency says that after three years of investigation, it is still unable to verify whether Iran's nuclear aims are purely peaceful.
The IAEA says Iran is moving towards large-scale uranium enrichment in defiance of international pressure.
The agency says although there is no evidence that Iran is seeking to develop nuclear weapons, a lack of full co-operation meant it could not reach a conclusion.
The former chief weapons inspector in Iraq, Hans Blix, says the UN's nuclear agency's problem was that it could not prove conclusively that Iran was not planning to develop nuclear weapons.
The report comes as Iranian officials travel to Russia today to resume talks on setting up a joint uranium venture with Moscow.
