By Mark Heinrich

VIENNA (Reuters) - An inquiry by the U.N. nuclear watchdog into alleged atom bomb research by Iran has degenerated into a silent standoff a few months after Tehran asserted "the matter is over," U.N. officials said on Wednesday.

"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 program," a senior United Nations official said.

The nuclear watchdog, the International Atomic Energy Agency (IAEA), meanwhile presented its latest report on Iran, saying it planned 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 its figures showed Iran had 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, U.N. officials said.

Iran, the world's No. 4 oil exporter, says its program is for generating more electricity. The West suspects a covert effort to develop the capacity to fuel nuclear weapons.

Analysts believe Iran could be as little as one or two years from enriching uranium to use in an atom bomb, if it so chose.

The report said Iran had stockpiled 630 kg (1,385 pounds) of low-enriched uranium so far. It would need 1,700 kg (3,740 pounds) to convert into enough high-enriched uranium (HEU) for an atomic bomb, experts estimate.

Mohammad Saeedi, the deputy head of Iran's Atomic Energy Organisation, told Reuters in Tehran the report showed Iran had "provided necessary access" for U.N. inspectors in 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 said.

But the report said that unless Iran produced credible evidence for its denials that it tried to "weaponise" nuclear materials, or permitted inspections beyond declared atomic sites, the IAEA could not verify Iran's enrichment was wholly peaceful.

"COMMUNICATION VACUUM"

The IAEA has struggled to get to the bottom of U.S. intelligence suggesting that Iran 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.

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.

The investigation has not advanced an inch since then, with both the agency and Iran standing their ground with arms crossed, said the U.N. officials, who asked for anonymity.

"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," said one senior official.

Though the IAEA's non-proliferation investigation is in the deep freeze, the agency continues dealing with Iranian officials with respect to routine inspections of Tehran's nuclear sites.

Iran says the U.S. intelligence is forged and sites the IAEA wants to visit are conventional military facilities that any nation would keep off-limits on security grounds. It argues they are therefore beyond the remit or competence of U.N. inspectors.

The IAEA report called on Iran to clarify procurement and research activities by military-related institutes and production of nuclear equipment by defense ministry firms.

In June, Iran said it had turned over more than 200 pages of documents to the IAEA which had answered all relevant questions and proclaimed: "The matter is over." The IAEA disagreed.

But Iran will face little pressure to change course before lame-duck U.S. President George W. Bush leaves office in January and president-elect Barack Obama takes over, diplomats say.

Senior officials of six world powers discussed Iran on Paris on November 13 but again failed to agree any initiatives such as tougher U.N. sanctions sought by the U.S., France and Britain in addition to three rounds of measures already imposed.

China and Russia dispute the need for punishing and isolating Iran, believing it poses no present threat to peace and only more concerted diplomacy will achieve a solution.

(Additional reporting by Parisa Hafezi in Tehran; editing by Mark Trevelyan)

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