Vienna - Iran has begun installing advanced centrifuges at its main uranium enrichment plant, the United Nations nuclear watchdog said on Thursday, a defiant step likely to anger world powers ahead of a resumption of talks with Tehran next week.

In a confidential report, the International Atomic Energy Agency (IAEA) said 180 so-called IR-2m centrifuges and empty centrifuge casings had been hooked up at the plant near the central town of Natanz. They were not yet operating.

If launched successfully, such machines could enable Iran to significantly speed up its accumulation of material that the West fears could be used to devise a nuclear weapon. Iran says it is refining uranium only for peaceful energy purposes.

It was not clear how many of the new centrifuges Iran aims to install at Natanz, which is designed for tens of thousands. An IAEA note informing member states late last month about Iran's plans implied that it could be up to 3 000 or so.

Iran has for years been trying to develop centrifuges more efficient than the erratic 1970s IR-1 model it now uses, but their introduction for full-scale production has been dogged by delays and technical hurdles, experts and diplomats say.

The deployment of the new centrifuges underlines Iran's continued refusal to bow to Western pressure to curb its nuclear programme, and may further complicate efforts to resolve the dispute diplomatically, without a spiral into Middle East war.

Iran has also started testing two new centrifuge models, the IR-6 and IR6s, at a research and development facility, the IAEA report said. Centrifuges spin at supersonic speed to increase the ratio of the fissile isotope in uranium.

Six world powers and Iran are due to meet for the first time in eight months in Kazakhstan on February 26 to try again to break the impasse but analysts expect no real progress toward defusing suspicions that Iran is seeking nuclear weapons capability.

In a more encouraging sign for the powers, however, the IAEA report said Iran in December resumed converting some of its uranium refined to a fissile concentration of 20 percent to powder for the production of reactor fuel.

That helped restrain the growth of Iran's higher-grade uranium stockpile since the previous report in November, a development that could buy more time for diplomacy and delay possible Israeli military action.

The report said Iran had increased to 167kg its stockpile of 20-percent uranium - a level it says it needs to make fuel for a Tehran research reactor but which also takes it much closer to weapons-grade material if processed further.

One diplomat familiar with the report said this represented a rise of about 18-19kg since the November report, a notable slowdown from the previous three-month period when the stockpile jumped by nearly 50 percent after Iran halted conversion.

About 240-250kg of 20 percent enriched uranium is needed for one atomic bomb if refined to a high degree.

Israel, which has warned it might bomb arch-enemy Iran's nuclear sites as a last resort, last year gave a rough deadline of mid-2013 as the date by which Tehran could have enough higher-grade uranium to produce a single atomic bomb.

But a resumption of conversion, experts say, means the Israeli “red line” for action could be postponed.

Refined uranium can fuel nuclear energy plants, which is Iran's stated aim, or provide the core of an atomic bomb, which the United States and Israel suspect may be its ultimate goal.

The United States, Russia, China, France, Britain and Germany are to meet Iran for negotiations in Kazakhstan on February 26 to tackle a row that has already produced four rounds of UN sanctions against Iran, as well as a European oil embargo.

They want Iran to halt its 20-percent enrichment and shut the Fordow underground plant where this work is carried out.

Iran wants them to recognise what it regards as its right to refine uranium for peaceful purpose and to relax sanctions battering its oil-dependent economy.

In Paris, French deputy foreign ministry spokesperson Vincent Floreani said the powers were ready to make a new offer to Iran and that they hoped Tehran would engage seriously in the talks.

“We will make a new offer that will have significant new elements,” Floreani said. “The approach... is to begin gradually with confidence-building measures. We want a real exchange that will lead to concrete results.”

The IAEA report said Iran had informed the UN agency during an inspection of the Bushehr nuclear power plant in mid-February that the reactor was shut down, giving no details. The Russian-built plant on Iran's Gulf coast is the Islamic state's first nuclear energy station, but has been plagued by delays. - Reuters