The United States on Monday stepped up criticism of Iranian plans to build 10 more uranium enrichment plants on with UN ambassador Susan Rice labeling them "unacceptable."

And in a further sign that US patience with Tehran is wearing thin, Rice warned that the dual-track approach of President Barack Obama's administration on Iran could soon shift from engagement to "pressure."

"We view the Iranian announcement, if it is in fact accurate and implemented that they intend to build 10 additional facilities, as completely inappropriate and further isolating Iran from the international community. We view that frankly as unacceptable," she said.

"And while we have been and will remain in close consultation with our P5+1 partners on the way forward, we have said that this is a dual track effort."

The P5+1, consisting of permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany, has been negotiating for months with the Islamic republic over its suspect nuclear activities.

The powers have long suspected that Iran, despite its fierce denials, is trying to build a nuclear bomb. They object to Tehran's uranium enrichment work which can be used to power nuclear reactors, but in highly purified form it can make the fissile core of an atom bomb.

"As Iran makes choices that seem to indicate that it is not at this stage ready and willing to take up the offers on the engagement track, then we will put greater emphasis on the pressure track," Rice said.

"Time is short and we are serious about implementing to the fullest extent that dual track policy."

The UN ambassador said Obama and other P5+1 leaders had made it quite clear that they would "take stock at the end of the year and see where we are," but that Iran must take advantage of "very concrete and constructive offers."

While Britain, France, Germany and the United States have given a strong reaction to Iran's latest announcement on Sunday and threatened new sanctions, Russia warned against escalating the nuclear row. There was no word from the Chinese.

Backing for new sanctions from China and Russia, close trade partners with Iran, would be key to imposing new penalties.

Iran, already under three sets of UN sanctions for defiantly enriching uranium at its Natanz facility, further infuriated world powers in September when it disclosed it was building the Qom plant.

World powers are also irked at Tehran for refusing a UN-brokered nuclear fuel deal, which envisages shipping abroad Iran's low-enriched uranium (LEU) for conversion into 20 percent enriched uranium to fuel a medical research reactor in Tehran.

Iran insists it is ready to send its LEU abroad only if there is a simultaneous exchange of fuel inside the country.

The defiant Iranian government led by President Mahmoud Ahmadinejad vowed on Sunday to build 10 new uranium enrichment plants, state media reported.

The hardline stance -- seen as hitting out at world powers led by Washington -- came after the conservative-dominated parliament urged the government to reduce ties with the UN atomic watchdog which on Friday condemned Iran for building its second uranium enrichment plant.