U.N. inspectors probing Iran's nuclear program have found equipment that can enrich uranium for weapons use and is far more advanced than anything Tehran (search) has previously acknowledged, diplomats said Thursday.

The find of the advanced centrifuge system is the second piece of evidence uncovered this month that casts new doubt on Iran's commitment to prove it does not want atomic weapons.

White House spokesman Scott McClellan (search) said the reported discovery raises "serious concerns."

"A country with the vast oil and gas resources of Iran has no legitimate need for nuclear energy," McClellan told reporters. "And full confidence about Iran's nuclear program requires Iran to abandon uranium enrichment and reprocessing activities."

A U.S. official, speaking on condition of anonymity, called the latest find "the second dramatic disclosure" in a row. "We knew that the Iranians were not completely clean, but we were willing to give them the benefit of the doubt."

Iran insisted its intentions are peaceful and that its centrifuges are to process uranium for nuclear power, not warheads. Without explicitly acknowledging the discovery, Iranian Foreign Ministry (search) spokesman Hamid Reza Asefi said any advanced "P-2" centrifuge system in the country was not in use but rather at a research stage.

One of the diplomats said the centrifuge was apparently located at an Iranian air base outside the capital — which would strengthen the arguments of the United States and other nations that Tehran is trying to make weapons.

But several other diplomats said they did not know where the equipment was found, and the Iranian government said there were no nuclear projects on any military base in the country.

Confronted by evidence last year, Iran acknowledged hiding nearly two decades of nuclear activity, including importing enrichment technology linked to the black market network of Pakistani scientist Abdul Qadeer Khan (search).

Those imports of know-how have allowed Tehran to create a domestic production line of centrifuges that can be used to process uranium for power — or for warheads at highly enriched level.

Under international pressure last year, it pledged to cooperate fully with the International Atomic Energy Agency in efforts to prove it was not interested in nuclear weapons, including opening its activities to full outside perusal.

Since then, however, the Vienna-based nuclear agency has dug up evidence that indicates continued Iranian secrecy.

The advanced centrifuge system whose existence was revealed Thursday appeared linked to drawings found about a week ago by nuclear inspectors of the P-2, a centrifuge far more advanced than the thousands of P-1s Iran now acknowledges having.

Diplomats last week told The Associated Press that preliminary IAEA investigations indicated the drawings matched those of equipment found in Libya and supplied by the Pakistani network headed by Khan.

The International Atomic Energy Agency (search) said it had no comment ahead of a report expected by early next week on Iran's nuclear program that is being drawn up for discussion at a top-level agency meeting next month.

At that meeting, the 35-member IAEA board of governors will also discuss a progress report on Libya, which late last year acknowledged trying to develop weapons of mass destruction and pledged to scrap them.

Unlike Libya, Iran denies ever having had such ambitions.

Finds of centrifuges and centrifuge drawings call into question Iran's pledge to be fully open, but they do not necessarily constitute evidence that Tehran is trying to make nuclear weapons.

Iran suspended its enrichment program last year but continues to make and assemble centrifuges despite international criticism that such actions violate the spirit of its pledge to stop all enrichment activities.

The IAEA, along with the United States and other nations, also want Iran to scrap its enrichment program instead of just suspending it — something Tehran refuses to do.

On Sunday, Foreign Minister Kamal Kharrazi said Iran intends to sell nuclear reactor fuel internationally, a move that would require restarting its nuclear enrichment program,

Kharrazi suggested that Iran may soon resume enriching uranium.

"We have taken a voluntary decision for confidence building and suspended (uranium) enrichment, but this does not mean that we will give up this industry, which is our national pride," Iran's official news agency quoted him as saying.