The United States is blasting Iran's decision to resume constructing machinery designed to enrich uranium, a process that can be used to generate weapons-grade nuclear material.

White House spokesman Scott McClellan says Iran's decision to resume building centrifuges can only heighten international concerns about its nuclear program. Speaking with reporters en route to a NATO summit in Turkey, Mr. McClellan said Iran's failure to comply with the International Atomic Energy Agency and to stop all enrichment-related reprocessing activities reinforces U.S. concern.

For the Bush administration, those concerns are that Iran's nuclear ambitions extend beyond peaceful energy purposes.

"We have been - the United States - the most certain about our views that the Iranians are trying to acquire military uses for nuclear power, maybe even nuclear weapons," said National Security Advisor Condoleezza Rice, spoking on the U.S. television program Fox News Sunday. "And that is why we have been the leaders in working with the IAEA, working with the Europeans to try and make certain that the Iranians know that they really only have two choices: one is to cooperate; the other is to face isolation."

Ms. Rice added that Iran's actions demonstrate why in 2002 President Bush labeled the country as part of a so-called "axis of evil" that included Iraq under Saddam Hussein as well as North Korea.

Iran has noted that constructing centrifuges is not the same as enriching uranium. Iran's decision to restart construction followed last week's resolution from the IAEA deploring what it described as Iran's failure to cooperate fully with IAEA inspectors. Iran has long insisted its nuclear program is not intended for military purposes.

But the head of the International Atomic Energy Agency, Mohamed ElBaradei, says international confidence in Iran's assurances, in his words, "has been shattered." Speaking with reporters in Moscow, Mr. ElBaradei said he hopes Iran will reverse its decision on centrifuge production.