The International Atomic Energy Agency, the United Nations' nuclear watchdog, said Monday it was worried Tehran was still hiding information about alleged studies into making nuclear warheads and defying UN demands to suspend uranium enrichment.

"The director general of the IAEA, Dr (Mohamed) ElBaradei, has produced another thorough report on Iran's nuclear activities," Miliband said in a statement.

"Once again it confirms that Iran has failed to suspend enrichment-related activities, has made no progress on the transparency measures the UN Security Council and IAEA have long called for, and has failed to answer the IAEA's questions relating to studies with a possible military dimension.

"Dr ElBaradei says that this is a 'matter of serious concern'. Iran needs to provide answers immediately, and come clean about its past activities. There is no justification for further delay.

"This is critical for international trust to be restored, as is suspension of enrichment-related activities."

Meanwhile in Tehran, Iran's new parliament speaker Ali Larijani on Wednesday warned the IAEA that the country could revise its cooperation with the body.

Britain and the other four permanent members of the Security Council -- China, France, Russia and the United States -- plus Germany are locked in carrot-and-stick negotiations with Iran over its nuclear programme.