Iran has amassed more than two tonnes of enriched uranium, the UN nuclear watchdog says in a report that has heightened Western concerns about the country developing the ability to produce a nuclear weapon.

Two tonnes of uranium would be enough for two nuclear warheads, although Iran says it does not want weapons and is only pursuing civilian nuclear energy.

Iranian President Mahmoud Ahmadinejad speaks during a ceremony marking the fourth national anniversary of nuclear technology in Tehran April 9, 2010. Iran has amassed more than two tonnes of enriched uranium, the UN nuclear watchdog said in a report released Tuesday. (Morteza Nikoubazl/Reteurs) The United States and the four other permanent UN Security Council members — Russia, China, Britain and France — have tentatively backed a draft fourth set of UN sanctions against Iran over its refusal to stop enriching uranium.

For seven months, Iran refused to accept a deal brokered by the IAEA that foresaw Iran exporting 1,200 kilograms of low-enriched uranium to Russia and France to be turned into fuel for Tehran's research reactor.

The West backed that offer because it would have committed Iran to exporting most of the enriched uranium it had produced and left it with less than the 1,000 kilograms of material needed to produce enough weapons-grade uranium for a bomb.

Iran rejected the offer then but now says it is ready to ship out the same amount of material and has enlisted the backing of Turkey and Brazil in trying to reach a compromise and derail the new sanctions push.

International concerns rise

Iran insists it has no interest in nuclear weapons. But its refusal to stop enrichment — which can create both nuclear fuel and warhead material — and its stonewalling of IAEA efforts to investigate suspicions it is interested in developing such arms have increased international concerns.

The restricted IAEA report said that the UN watchdog agency "remains concerned about the possible existence in Iran of past or current undisclosed nuclear related activities, involving military related organizations, including activities related to the development of a nuclear payload for a missile."

On enrichment, the report made available to the AP shortly after release to the UN Security Council and the IAEA's 35-nation board said Iran had now enriched 2,427 kilograms to just over three per cent level.

That means shipping out 1,200 kilograms now would still leave Iran with more than enough material to make a nuclear weapon. That makes the deal unattractive to the U.S and its allies.

The report confirmed that Iran continues a separate program of small-scale enrichment of uranium, using 3.5 per cent feedstock and enriching to near 20 per cent — another hurdle for the West. Iran could produce weapons grade uranium much more quickly from the 20 per cent level, making the separate program another hurdle to any fuel swap deal.

Suspicion in the West

The U.S. and its allies view Tehran's insistence on continuing higher enrichment even as it offers to accept the swap deal with suspicion since it originally said it had to enrich to 20 per cent as the first step in making fuel for the Tehran research reactor.

The IAEA also said that equipment had been removed from a laboratory it was investigating, confirming a report last week to the AP from diplomats familiar with the issue.

At issue is pyroprocessing, a procedure that can be used to purify uranium metal used in nuclear warheads.

In January, Iran told the agency that it had carried out pyroprocessing experiments, prompting a request from the nuclear agency for more information — but then backtracked in March and denied conducting such activities.

IAEA experts last month revisited the site — the Jabr Ibn Jayan Multipurpose Research Laboratory in Tehran — only to establish "that the electrochemical cell had been removed" from the unit used in the experiments, the report said.