An Iranian technician works at the Isfahan Uranium Conversion Facilities (UCF), 420 kms south of Tehran (File)

The United Nations nuclear agency says it remains seriously concerned about allegations that Iran studied how to make nuclear weapons.

The International Atomic Energy Agency says in a new report that Iran has not provided "substantive explanations" to refute the accusations.

The U.N. agency says Iran should hand over more information about its previous testing of high explosives and missile-related work.

Western intelligence agencies suspect the activities were part of Iranian studies about making atomic weapons. Iran dismisses that accusation as "baseless."

The U.S. envoy to the IAEA, Gregory Schulte, says the report shows in "great detail" how much Iran needs to explain and "how little it has," as he put it. European officials also criticized Iran for not providing sufficient answers to U.N. nuclear inspectors.

The Iranian envoy to the IAEA, Ali Ashgar Soltanieh, says the report proves that Iran's nuclear activities are peaceful.

The IAEA report also notes that Iran continues to defy U.N. demands to suspend uranium enrichment, despite three rounds of U.N. Security Council sanctions.

The report says Iran is now operating 3,500 uranium-enriching centrifuges at its main nuclear site in Natanz, an increase of about 500 centrifuges from February.

The United States and its Western allies accuse Iran of trying to develop nuclear weapons under cover of a civilian energy program, a charge Iran denies.