Iran has doubled its capacity at an underground nuclear facility, a UN atomic agency report showed Thursday, in spite of UN Security Council resolutions, sanctions and talk of Israeli military action.

The International Atomic Energy Agency also said that its ability to inspect the Parchin military base where it suspects Iran did nuclear weapons research in the past had been "significantly hampered" by a suspected clean-up.

The IAEA said it was still "unable to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities."

The restricted report, circulated to IAEA member states and seen by AFP, showed that Iran now has around 2,000 uranium-enrichment centrifuges installed, compared with around 1,000 in May, at Fordo.

Enriched uranium can be used for peaceful purposes but also, at highly concentrated purities, for nuclear weapons, and multiple UN Security Council resolutions have called on Iran to suspend enrichment.

Iran's supreme leader Ayatollah Ali Khamenei told a summit of the Non-Aligned Movement in Tehran on Thursday that his country "is never seeking nuclear weapons" and would "never give up the right to peaceful nuclear energy."

Fordo is dug into a mountain near the holy city of Qom and mostly enriches to 20 percent purities, technically speaking a short hop from the 90-percent needed in a nuclear weapon.

The number of centrifuges operating remained around 700, however, and monthly production is unchanged at around 15 kilos of 20-percent uranium per month. It is also unclear whether the newly installed machines will also enrich to 20 percent.

A senior official familiar with the IAEA investigation said on condition of anonymity that it was unclear whether technical problems were behind no more centrifuges being operational.

But Mark Fitzpatrick, from the International Institute for Strategic Studies think-tank in London, saw an Iranian attempt to strengthen its negotiating position in currently stalled talks with world powers.

"It doesn't have a current need to increase the production rates but it does want to have the capacity in place to do so in the future," he told AFP.

Iran's stated reason for enriching to 20 percent is for fuel for a medical research reactor in Tehran, but the official said that it "now has already produced enough material for at least 10 years of operation."

The IAEA also wants Iran to address what it believes is evidence that until 2003, and possibly since, Tehran had a structured programme of research into nuclear weapons.

Iran has flatly rejected these claims, set out in a major IAEA report last November, and says it will only give the agency the desired access as part of a broader agreement governing its future relations with the watchdog.

In particular the IAEA wants to be able to visit the Parchin military base near Tehran, where it believes Iran conducted explosives tests for nuclear warhead designs.

Western nations accuse Tehran of "sanitising" the site to remove evidence and in the new report the IAEA said Iranian activities "will significantly hamper the Agency's ability to conduct effective verification."

Citing satellite images, these included the demolition of certain buildings, "significant ground scraping and landscaping" over an extensive area and the "shrouding" of buildings, the IAEA said.

The latest in a series of meetings aimed at persuading Iran to give the IAEA access to documents, scientists and sites involved in this alleged drive to get the bomb failed last Friday.

A separate, higher-level renewed diplomatic push in talks between Iran and the "P5+1" five permanent members of the UN Security Council -- the US, China, Russia, France and Britain -- plus Germany appears stuck, meanwhile.

The P5+1 chief negotiator, EU foreign policy chief Catherine Ashton, will talk with Iranian counterpart Saeed Jalili "in the coming days," Ashton's spokesman said Thursday. He did not say whether they would meet in person.

"The window of opportunity to resolve this remains open ... but it will not remain open indefinitely," White House spokesman Jay Carney said in Washington on Thursday.