Iran has moved more centrifuge machines for enriching uranium to an underground facility offering better protection from possible airstrikes, the country's vice president said Monday.

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.

Abbasi, who is also Iran's nuclear chief, did not say how many centrifuges have been moved to Fordo or whether the machines installed are the new, more efficient centrifuges Iran has promised or the old IR-1 types.

Iran previously moved some of its centrifuges to an underground enrichment facility in Qoms. The decision to use multiple facilities on Iran's part renders the enrichment program more diffusible and any attack on it more complicated.

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 bomb.

The United States and Western 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 aimed at generating electricity, not a nuclear bomb.

But the International Atomic Energy Association (IAEA) says Iran has actively impeded its attempts to inspect its nuclear facilities under the Non-Proliferation Treaty (to which Iran is a signatory). Previously, the IAEA reported the Islamic republic had attempted to obtain nuclear technologies to create an implosion device, considered more advanced than the bomb America used on Hiroshima.

Iran has been enriching uranium to less than 5% for years, but it began to further enrich its uranium stockpile to nearly 20% in February 2010, saying it needs the higher grade material to produce fuel for a Tehran reactor that makes medical radioisotopes needed for cancer patients.

However, Iran's higher-grade enrichment efforts are of particular concern to the West because 20% percent enrichment is a significant step towards what is needed for weaponization.

Iran reportedly has over 3,600 kilograms of enriched uranium at 3.5 percent, enough for three nuclear bombs if enriched further. It also has an announced inventory of 40 kilograms of 20 percent enriched uranium. If true, it would take Iran 2-3 months to further enrich their stockpile into 20 kilograms of enriched uranium over 90 percent, sufficient for one nuclear warhead.