US says transfer of centrifuges to an underground uranium enrichment site raises suspicions.
Iran has moved some of its centrifuges to an underground uranium enrichment site that offers better protection from possible air strikes, the country's vice president said.
Engineers are "hard at work" preparing the facility in Fordo, which is carved into a mountain to protect it against possible attacks, to house the centrifuges, Fereidoun Abbasi was quoted as saying by state TV on Monday.
Abbasi, who is also Iran's nuclear chief, did not say how many centrifuges have been moved to Fordo nor whether the machines installed are the new, more efficient centrifuges Iran has promised or the old IR-1 types.
He did specify that the centrifuges will be taken to Fordo from Iran's main uranium enrichment facility in Natanz, central Iran.
Uranium enrichment lies at the heart of Iran's dispute with the West, a technology that can be used to produce nuclear fuel or materials for atomic bombs.
The United States and some of its allies accuse Iran of using its civilian nuclear program as a cover to develop atomic weapons. Iran has denied the charges, saying its nuclear program is peaceful and solely aimed at generating electricity.
On Monday, US State Department spokeswoman Victoria Nuland said the new programme raises suspicions.
"The Iranian nuclear programme offers no plausible reasons for its existing enrichment of uranium up to nearly 20 per cent, nor ramping up this production, nor moving centrifuges underground," she said.
"And its failure to comply with its obligations to suspend its enrichment activities up to 3.5 per cent and nearly 20 per cent have given all of us in the international community reason to doubt its intentions."
Iran has been enriching uranium to less than five per cent for years, but it began to further enrich its uranium stockpile to nearly 20 per cent as of February 2010, saying it needs the higher grade material to produce fuel for a Tehran reactor that makes medical radioisotopes for cancer treatment.
Weapons-grade uranium is usually about 90 per cent enriched.
Iran's higher-grade enrichment efforts are of particular concern to the West because uranium at 20 per cent enrichment can be converted into fissile material for a nuclear warhead much more quickly than that at 3.5 per cent.
Abbasi said Tehran was in no rush to install the centrifuges and that experts are observing all technical standards.
In June, Abbasi said Iran plans to triple its output of the 20 percent enriched uranium and move the entire program to the new, secretly-built Fordo facility, just north of the holy city of Qom in central Iran.