VIENNA (Reuters) - Iran has begun installing advanced centrifuges at its main uranium enrichment plant, a U.N. nuclear report 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.

A general view of the Bushehr nuclear power plant, some 1,200 km (746 miles) south of Tehran October 26, 2010. REUTERS/IRNA/Mohammad Babaie

If operated 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.

The report also said Iran had increased to 167 kg (367 pounds) its stockpile of uranium refined to a fissile purity of 20 percent - a level it says it needs for conversion into reactor fuel. About 240-250 kg of 20 percent enriched uranium is needed for one atomic bomb if refined to a high degree.

Iran resumed converting higher-grade enriched uranium for fuel production in December and had since fed 28.3 kg of the material for this stated purpose, the report added.

It further said that "extensive" activities - an allusion to clean-up and renovations - at Iran's Parchin military site would seriously undermine an IAEA investigation to determine whether explosives research relevant to nuclear weapons was done there.

(Reporting by Fredrik Dahl; Editing by Mark Heinrich)

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