The UN nuclear watchdog has said it believes Iran is still withholding information on its nuclear programme.

In its report, the IAEA said Tehran's alleged weapons development studies remain a matter of serious concern.

It added that Iran is operating 3,500 centrifuges, the machines used to enrich uranium, at its plant at Natanz.

Enriched uranium can be used in nuclear weapons, but Tehran insists its nuclear programme is entirely peaceful.

Last month, the IAEA said it had reached an agreement with Iranian officials to clarify the main outstanding question about Iran's past nuclear work by the end of May. But in its latest report, the IAEA said that Tehran should provide much more explanation and information on its nuclear activities.

Iran told the IAEA it plans to have 6,000 uranium-enriching centrifuges operating by the end of the summer.

The suspension of uranium enrichment is a key demand of the UN Security Council, which has imposed sanctions on Iran.

Iran insists it has a right to enrich uranium under the Nuclear Non-proliferation Treaty and says it want only to generate power.

Iran's nuclear programme has been under IAEA investigation since 2002, when Iranian dissidents revealed the existence of secret uranium enrichment.