Iran intends to install more modern equipment at Natanz, one of its main nuclear sites, according to a document seen by AFP on Thursday.

The UN atomic agency document said that Iran informed it in a letter dated January 23 that "centrifuge machines type IR2m will be used in Unit A-22" at the Fuel Enrichment Plant (FEP) at Natanz.

The International Atomic Energy Agency replied in a letter dated January 29 asking for more information on the announcement, which comes despite international sanctions aimed at slowing Iran's nuclear programme.

Natanz in central Iran is currently used to enrich uranium, a process at the heart of the international community's concerns about Iran's nuclear programme, mostly using older model IR-1 machines, which operate more slowly.

A Vienna diplomat told AFP on condition of anonymity that the new machines would likely be used to enrich uranium to fissile purities of five percent.

Of greater concern than Natanz is Iran's Fordo site, which enriches uranium to 20-percent purities, significantly closer to the 90-percent level needed for a bomb.

"So far they are mostly only enriching to five percent at Natanz, so it would be a surprise if Iran came back and said it was for 20 percent," the diplomat said.

Enriched uranium can be used for peaceful purposes such as in atomic power stations -- Iran currently has one functioning plant -- but also, in highly-enriched form, for a nuclear bomb.

Because of concerns that Iran may be seeking to develop nuclear weapons, something it denies, numerous UN Security Council resolutions have called on Tehran to suspend all enrichment activities.

Several rounds of sanctions have been imposed on Iran aimed at preventing it procuring and developing technology for its nuclear activities.

Unilateral sanctions imposed in 2012 by the United States, the European Union and others have also targeted Iran's vital oil exports, leading to major economic problems.

The IAEA declined to comment on Thursday. The Vienna-based agency is due to release its latest quarterly updated on Iran's atomic activities on February.