VIENNA  - Iran is trying to accelerate its uranium enrichment programme, a UN nuclear report showed, but experts said it was unclear when Tehran's new machines could start operating and how efficiently they would work.

The Islamic state's progress in introducing next-generation centrifuges is closely watched in the West and Israel as it would enable Tehran to speed up accumulation of material that could be used to build atomic bombs. Iran denies any such aim.

Iran has tried for years to develop centrifuges more advanced than the erratic 1970s-vintage IR-1 machines it now runs, but deploying new models has been dogged by technical hurdles and difficulty in obtaining parts abroad.

It is now pressing ahead with installing a more efficient version known as the IR-2m at its main enrichment plant near the central town of Natanz, according to the report by the International Atomic Energy Agency (IAEA). It has also put in place another centrifuge type, the IR-5, for the first time at a research and development facility at Natanz, joining five others being tested there, the report issued to member states late on Wednesday said. The rapid installation of one of them, the IR-2m, at a Natanz production unit since it started earlier this year indicates that Iran can make such equipment, at least to some extent, despite tightening sanctions on the country. Critics say Iran is trying to achieve the capability to make atomic arms. Iran denies this, saying it needs nuclear power for energy generation and medical purposes and that it is Israel's reputed nuclear arsenal that threatens regional peace.

Iran's expansion of sensitive nuclear activity breaks UN resolutions and increases concerns about whether its nuclear programme is entirely peaceful, the European Union said on Thursday, reacting to the IAEA's findings. Pointing to the installation of advanced centrifuges and other developments in the Meanwhile, the country's envoy to the agency said on Thursday that the new report by the UN atomic watchdog validates Iran's progress in its "peaceful" nuclear activities despite international sanctions.

Ali Asghar Soltanieh, speaking to Fars news agency in Vienna, where the IAEA is based, said the report was proof of Iran's "technical and scientific success".

It also showed there was "no evidence of diversion in nuclear material and activities toward military purposes" and that "all the centrifuges and each gram of uranium are under the supervision of the agency," he said. In its report issued on Wednesday, the IAEA outlined further progress at a reactor under construction at Arak, in central Iran.

, which Western countries fear could provide the Islamic republic with plutonium if the fuel is reprocessed.

Highly enriched uranium and plutonium can both be used in a nuclear weapon. Soltanieh did not comment on the developments at the Arak reactor.

The IAEA report showed Iran has so far produced 324 kilograms (714 pounds) of 20-per cent enriched uranium, 44 kilograms more than three months ago, but that 140.8 kilograms had been diverted to fuel production, up from 111 kilograms.

Experts say about 240-250 kilograms are needed for one bomb.

The UN Security Council has passed numerous resolutions calling on Iran to suspend all enrichment and heavy water activities of the kind under development at Arak.

It has imposed four rounds of sanctions.

Last year additional unilateral US and EU sanctions targeting Iran's oil exports and its financial system began to cause real problems for the Islamic republic's economy.

Israel, the Middle East's sole if undeclared nuclear-armed state, has refused to rule out military action against Iran, as has US President Barack Obama. Iran insists its atomic activities are peaceful.

Parallel diplomatic efforts to resolve the impasse, most recently in six-power talks with Iran in Kazakhstan in April, have failed to make concrete progress.