"There is no nuclear site that the (International Atomic Energy) Agency is not aware of and that we have hidden from IAEA inspectors," foreign ministry spokesman Hamid Reza Asefi told reporters ahead of a key visit by the UN watchdog's director, Mohammed ElBaradei.

"We have a transparent and constructive cooperation with the agency, and this will continue," the spokesman said, describing ElBaradei's visit on Tuesday -- his third trip to Iran since February 2003 -- as "important".

"We are committed to our engagements, and up to now have not violated any of them," he said.

Iran is under mounting international pressure over its nuclear programme, and angrily denies US charges that its civil atomic energy programme is a cover for ambitions to secretly develop nuclear weapons.

In March it was condemned by the IAEA for continuing to hide sensitive nuclear activities, including designs for sophisticated P2 centrifuges for making enriched uranium which could be weapons-grade.

In addition, Britain, France and Germany have condemned Iran's decision to resume work on a key part of the nuclear fuel cycle.

In a deal with the IAEA brokered last year by the European Union's big three, Tehran agreed to suspend uranium enrichment and related activities while UN inspectors delved into its programme.

But Asefi claimed the resumption of work at a uranium conversion facility near the central city of Isfahan, announced here last week, was not a violation of the deal.

"The resumption of activities at Isfahan does not violate any of our commitments. It is for the production of UF4 in an experimental fashion, which is nothing to do with our commitment to voluntarily suspend uranium enerichment," he told a weekly news conference.

UF4, or, uranium tetrafluoride, can serve as a base for the production of uranium hexafluoride (UF6) -- a compound then used in enrichment.

The problem appears to centre on how each side defines the suspension, with Iran apparently working to a more narrow definition of what constitutes a part of the enrichment process.

On the other hand, the Europeans had hoped Iran would entirely halt its work on the highly sensitive nuclear fuel cycle -- which is allowed under the terms of the nuclear Non-Proliferation Treaty (NPT) but once mastered can easily be diverted towards military ends.

But Asefi said the Europeans should instead work on honouring their engagements under the NPT -- which commits member states to providing peaceful nuclear assistance among themselves.

"Setting new demands will only damage confidence building," he argued.

The foreign ministry spokesman also confirmed reports from IAEA headquarters in Vienna that a new visit by inspectors would take place later this month, with the latest probe focussing on "the suspension of enrichment and the P2" centrifuge.

Diplomats in Vienna said the mission would also be looking into whether Iran had acquired weapons designs from a nuclear black market that is being unravelled following Libya's decision to abandon its efforts to develop weapons of mass destruction.

The IAEA has been investigating since February 2003 whether Iran's nuclear programme is peaceful, as the country insists.

The body is to report its latest findings at a meeting in Vienna in June.

An IAEA ruling that Iran is in non-compliance with the NPT would send the issue to the UN Security Council, which could then impose sanctions on the Islamic republic.

WAR.WIRE