EU foreign ministers urged Tehran to heed a tough resolution by the International Atomic Energy Agency (IAEA) on March 13 that rapped the country for hiding sensitive nuclear activities in an October report to the UN body.
In a statement, the ministers "urged Iran to provide full and proactive cooperation with the Agency in resolving all such questions in a spirit of full transparency".
They added they expected "Iran to cooperate with the Agency fully and to ensure that all future inspections can take place without delay".
But the EU also said it welcomed Iran's February decision to extend the scope of its suspension of uranium enrichment and reprocessing activities.
The IAEA has since February 2003 been verifying whether Iran's nuclear programme is peaceful, or devoted to secretly developing atomic weapons, as the United States claims.
The EU, which favours "constructive engagement" with Iran, has been more cautious in its appraisal of Tehran's nuclear drive.