The International Atomic Energy Agency says it has not resolved questions about a possible military dimension to Iran’s nuclear programme.
In its latest report, the UN nuclear watchdog said it had failed to make meaningful progress in assessing Iran’s past nuclear activities.
Iran was also continuing to enrich uranium in defiance of a UN Security Council resolution, it said.
The US said Iran could face further sanctions unless it changed course.
Iran says its nuclear programme is aimed solely at civilian atomic energy, but Western nations accuse Iran of seeking to develop a nuclear weapon.
‘No credible assurances’
In its report, the IAEA said that Iran was failing to co-operate with its investigators.