The White House warned Thursday that Iran's decision to install more modern equipment at its Natanz nuclear plant was a "further escalation" in the showdown over its atomic program.

"The installation of new advanced centrifuges is a further escalation and a continuing violation ... of Iran's obligations under relevant United Nations Security Council and IAEA board resolutions," White House spokesman Jay Carney said.

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

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.

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.