Iran's defiance will likely heighten tensions with the West, which has signaled it is running out of patience with Iran's continuing enrichment and its balking at a U.N. deal aimed at ensuring Tehran cannot build a nuclear weapon in the near-term future. The U.S. and its allies have hinted at new U.N. sanctions if Iran does not respond.
The White House said the move "would be yet another serious violation of Iran's clear obligations under multiple U.N. Security Council resolutions and another example of Iran choosing to isolate itself."
"Time is running out for Iran to address the international community's growing concerns about its nuclear program," White House press secretary Robert Gibbs said.
British Foreign Secretary David Miliband described Iran's move as a provocation.
"This epitomizes the fundamental problem that we face with Iran," he said. "We have stated over and again that we recognize Iran's right to a civilian nuclear program, but they must restore international confidence in their intentions. Instead of engaging with us Iran chooses to provoke and dissemble."
On Friday, the U.N.'s International Atomic Energy Agency issued a strong rebuke of Iran over enrichment, infuriating Tehran. Parliament speaker Ali Larijani threatened on Sunday to reduce cooperation with the IAEA.
"Should the West continue to pressure us, the legislature can reconsider the level of Iran's cooperation with the IAEA," Larijani told parliament in a speech carried live on state radio.
Vice President Ali Akbar Salehi, who is also Iran's nuclear chief, said Sunday's decision was "a firm message" in response to the IAEA. He told state TV that the agency's censure was a challenge aimed at "measuring the resistance of the Iranian nation."
Any new enrichment plants would take years to build and stock with centrifuges. But the ambitious plans were a bold show by Iran that it is willing to risk further sanctions and won't back down amid a deadlock in negotiation attempts.
Iran currently has one operating enrichment facility, at the central town of Natanz, which has churned out around 1,500 kilograms (3,300 pounds) of low-enriched uranium over the past years -- enough to build a nuclear weapon if Iran enriches it to a higher level. Iran says it has no intention of doing so, insisting its nuclear program aims only to generate electricity.
The revelation of a second, previously unannounced facility, under construction for years at Fordo near the holy of Qom, raised accusations from the United States and its allies that Iran was trying expand enrichment in secret out of inspectors' sight. Iran denied the claim.
On Sunday, a Cabinet meeting headed by President Mahmoud Ahmadinejad ordered the Atomic Energy Organization of Iran to begin building five uranium enrichment plants at sites that have already been studied and propose five other locations for future construction within two months, the state news agency IRNA reported. All would be at the same scale as Natanz.
The new sites are to be built inside mountains to protect them from possible attacks, said Salehi, Iran's nuclear chief. They will also use a new generation of more efficient and more productive centrifuges that Iran has been working to construct, he and Ahmadinejad said.
In Vienna, spokeswoman Gillian Tudor said the U.N.'s International Atomic Energy Agency would have no comment on Tehran's announcement.
Israeli Prime Minister Benjamin Netanyahu called for "a concentration of sanctions and pressure on the Iranian regime, which is vulnerable economically" to rein in its nuclear ambitions. Israel has not ruled out military strikes against Iranian nuclear sites if its program is not stopped.
The IAEA censure against Iran on Friday was seen as a show of international unity behind demands that Tehran rein in its nuclear program -- though there does not yet appear to be consensus on imposing sanctions.
The IAEA resolution criticized Iran for secretly building the Fordo site and defying the U.N. Security Council call for a suspension of enrichment.
It noted that IAEA chief Mohamed ElBaradei cannot confirm that Tehran's nuclear program is exclusively geared toward peaceful uses, and expressed "serious concern" that Iran's stonewalling of an agency probe means "the possibility of military dimensions to Iran's nuclear program" cannot be excluded.
The U.N. seeks to stop Iran's enrichment, because the process can be used to produce either fuel for a reactor or a warhead. In the process, uranium gas is spun in centrifuges to be purified -- to a low degree for fuel, to a higher level for a bomb. Iran denies U.S. claims that it secretly aims to produce a nuclear weapon.
The United States and the top powers at the U.N. have been focused on winning Iran's acceptance of a deal under which it would ship abroad most of its low-enriched uranium stocks to be processed into fuel rods for a research reactor in Tehran. The move would leave Iran -- at least temporarily -- without enough uranium to produce a bomb.
But Iran has balked, presenting a counter-proposal with various changes. The West has demanded it accept the proposal as is.
In the wake of the IAEA rebuke, Iran has sought to signal that it can lash back if pushed. On Saturday, one hard-line lawmaker warned that parliament might withdraw the country from the Nuclear Non-Proliferation Treaty and stop all U.N. inspections -- a move that would sharply escalate the standoff with the West and cut off the U.N.'s only eyes on Iran's nuclear program.
But parliament took a lesser step on Sunday: 226 of the 290 lawmakers signed a letter urging the government to prepare a plan to reduce Tehran's cooperation with the IAEA in response to its resolution.
Iran touted the expansion of enrichment as necessary for its plans to generate 20,000 megawatts of electricity through multiple nuclear power plants in the next 20 years.
Ahmadinejad said 500,000 centrifuges will be needed in the new plants to produce between 250 to 300 tons of fuel annually, IRNA reported. About 8,600 centrifuges have been set up in Natanz, but only about 4,000 are actively enriching uranium, according to the IAEA. The facility will eventually house 54,000 centrifuges. The Fordo site is smaller, built for nearly 3,000 centrifuges.