Iran has now accumulated enough nuclear material to make an atomic bomb, weapons experts said today.

An analysis of the latest monitoring report on the country’s nuclear programme concluded Iran had amassed 630kg of low enriched uranium, enough for a single weapon.

Iran denies allegations it is seeking to build a bomb and claims it is only developing a peaceful energy programme in its plant at Natanz. Non-proliferation rules allow for a domestic nuclear industry.

But UN officials said they were unable to verify Iran's claims.

“We had gridlock before but until September at least we were talking to each other. Now it’s worse. There is no communication whatsoever, no progress regarding possible military dimensions in their programme,” said a senior official from the UN’s nuclear watchdog, the International Atomic Energy Agency.

The IAEA has struggled to get to the bottom of US intelligence suggesting that Iran has in the past melded projects to process uranium for atomic fuel, test high explosives at unusually high altitudes and revamp the cone of a long-range Shahab-3 missile in a way that would fit a nuclear warhead.

Iran says such intelligence is forged, but monitors have said that unless Iran produces credible evidence for its denials or permitted inspections beyond declared atomic sites, the IAEA could not verify Iran’s enrichment was wholly peaceful.

The last IAEA report, on September 15, detailed the Islamic Republic’s non-cooperation with requests for documents and access to sites and officials and physicists for interviews.

UN officials said that the situation has not since improved.

The only aspect of the inquiry that has continued are the routine inspections of the Natanz plant, the monitors said. These contacts had revealed Iranian plans to start installing another 3,000 centrifuges early next year, adding to 3,800 already enriching uranium and another 2,200 being gradually introduced.

But monitors also found that Iran has not boosted the number of centrifuges regularly refining uranium since reaching the 3,800 level in September. The reason for Iran’s relatively slow progress was unclear, UN officials said.

“Our questions are there and they need to be addressed. There is no point in writing them again every week. We are just awaiting their response,” one senior official said.

“But we have a long vacuum of communication now.”

Experts cautioned that although Tehran had enough material for a weapon, it was a largely symbolic milestone as Iran would have to convert the fuel into high enriched uranium and put it into a warhead design. Western analysts doubt Iran is technically capable building a functioning warhead.

“They clearly have enough material for a bomb,” said Richard Garwin, a top nuclear physicist who helped invent the hydrogen bomb and has been a Washington adviser for many years.

“They know how to do the enrichment. Whether they know how to design a bomb, well, that’s another matter.”

Mohammad Saeedi, the deputy head of Iran’s Atomic Energy Organisation, insisted that the report showed Iran had “provided necessary access” for UN inspectors within the framework of the nuclear Non-Proliferation Treaty and IAEA safeguards.

“Naturally in the future also the agency’s access and inspections within the same framework will continue,” he told reporters in Tehran.

Iran says that IAEA inspectors do not have the right to inspect the sites it is requesting access to, as they are conventional military facilities which any nation would keep off limits on security grounds.

It turned over more than 200 pages of documents to the IAEA in June and at the time insisted they answered all relevant questions, declaring: "The matter is over."

Diplomats say that despite Western refusal to accept such a stance, the Islamic Republic will face little pressure to change course before US President George Bush hands over to Barack Obama in January.

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