The United States yesterday expressed its concern over Iran's announcement that it was working on advanced P-2 centrifuges to enrich uranium.

"If the statements prove to be true, it would be a very serious concern," White House spokesman Scott McClellan told reporters.

Iranian President Mahmoud Ahmadinejad said last week that the Islamic republic was testing the P-2 centrifuge, a more sophisticated type.

Prior to the announcement, Ahmadinejad said that Iran had succeeded in enriching a small amount of uranium using a less-sophisticated type of centrifuge.

"Undisclosed work on P-2 centrifuges would be a further violation of Iran's safeguard obligations, in addition to those that have already been identified by the (IAEA) board," McClellan said.

"Such violations and failures by the regime to comply with its international obligations run contrary to the regime's claims that its nuclear program is solely for peaceful purposes," he said.

The United States has accused Iran of trying to develop nuclear weapons under the cover of civilian nuclear programs.

Iran, on the other hand, has reiterated that it is enriching uranium to a low degree to be used as fuel for generating power in a nuclear reactor.