In the presence of International Atomic Energy Agency inspectors, Iran on Tuesday unsealed uranium enrichment equipment that the U.N. agency had blocked from use because the Islamic republic was in violation of nuclear non-proliferation rules.

The return to its nuclear program at the plant in Natanz angered U.S. and European officials who say Iran is resuming nuclear research that they believe is part of an effort to build nuclear weapons.

Iran's move is a "serious escalation" of its nuclear standoff with the West, and if it continues to defy world opinion, the U.N. Security Council will have no choice but to impose sanctions, said White House Press Secretary Scott McClellan.

"Any resumption, any resumption of enrichment and reprocessing activities would be a further violation" of previous agreements made by Iran, McClellan said. "Such steps would be a serious escalation of the nuclear issue by the regime in Iran.

"If it continues down this road and the negotiations have run their course, then there is only one option to pursue. And that is referral to the Security Council. And that is what we will be talking with our — are talking about with our — European friends and others," McClellan said.

Iran would be committing a serious mistake if it ignored the international community on its nuclear program, said French President Jacques Chirac.

An IAEA statement issued in Vienna, Austria, said Iran did unseal the equipment, and told the agency the scale of its enrichment work would be limited.

"What we resume is merely in the field of research, not more than that," the deputy head of the Atomic Energy Organization of Iran, Mohammad Saeedi, told a news conference. "Production of nuclear fuel" — which would involve enrichment — "remains suspended," he said.

Saeedi refused to say whether the seals had been broken, calling it a "confidential issue between us and the IAEA."

McClellan said Iran's 18-year history of trying to hide weapons efforts has proven that the international community's worries about Tehran's nuclear efforts are "well founded."

"There is serious concern throughout the international community about the regime's behavior, and given Iran's history of concealing and hiding their nuclear activities from the international community and its continued non-compliance of its safeguard obligations, such concern is well-founded," he said.

Elsewhere in Washington on Tuesday, analysts following Iran's nuclear weapons intentions contended that Tehran never observed a "freeze" in its effort to gain a nuclear bomb and that Tuesday's move will put it even closer to its goal than previously known.

Strategic Policy Consulting chief Alireza Jafarzadeh, a FOX News contributor, alleged that Iran has manufactured as many as 5,000 centrifuges during the two-year timeframe that it claimed its activity was suspended. Jafarzadeh, an Iranian opposition activist who was responsible for exposing the Natanz facility in 2002, said he believes Tehran had pre-positioned these centrifuges for installation at Natanz before the "official" restart announced Tuesday and thus is much closer to weapons production than previously believed.

If Iran chooses to install these centrifuges, "it would put Iran only months away from having a nuclear bomb," he told a news conference held by the Iran Policy Committee.

Despite that threat, though, British Foreign Secretary Jack Straw said Tuesday that no military intervention against Iran is under consideration.

"This is a matter which has to be solved by peaceful means, but it will involve a good deal of diplomatic and other pressure on Iran. We have shown our good faith in Europe the past two and a half years. I do not believe we could have done more to reach out to the Iranians," he said.

Already, all five permanent members of the U.N. Security Council have each separately sent to Iran a diplomatic communiqué — known as a 'demarche' in international parlance — warning the Islamic Republic that it could face sanctions should it go forward with its nuclear program. The permanent members are the United States, the United Kingdom, France, Russia and China.

Bush administration officials told FOX News that the communiqués, sent Saturday, are an important step forward in blocking Iran's nuclear ambitions because China and Russia, which have huge economic interests in Iran, have hinted in the past that they would oppose and veto such a referral if brought to the Security Council.

"We are working very closely with Russia, China, France and Britain on sending a clear message to the Iranians," State Department spokesman Sean McCormack said.

Each of the permanent Security Council members has veto authority on the 15-member panel that enables them to thwart resolutions. Sources say the five sent separate communiqués because China and Russia repeatedly tried to soften the language of the warning and no consensus could be reached for a joint warning.

While pleased that China and Russia delivered separate warnings to Iran, Bush administration officials say they are very cautious about prematurely assuming that the two nations are willing to get tough with Iran and support sanctions.

"Given Iran's track record on seeking nuclear weapons under the cover of a civilian program, defying the international community, bobbing and weaving, obfuscating, we're ultimately all going to end up in the Security Council on this issue," said McCormack.

But one senior administration official suggested that both China and Russia sent their demarches reluctantly and might in the end oppose a referral or sanctions. Nonetheless, McCormack said the efforts by China and Russia are noteworthy.

"I think that the Chinese are perfectly capable of delivering their own messages," McCormack said. "What we have been doing, have done and will continue to do, is to continue to work with them, work with the Russians and others so that Iran receives a clear, consistent, unmistakable message from the rest of the world."

One senior official told FOX News that the demarches from each of the five permanent Security Council members urged Iran to resume talks with the EU-3, comprised of Germany, France and the U.K.

The United States is backing a stalled European effort to negotiate with Iran, and supports a separate offer from Russia to perform some of the most sensitive nuclear enrichment tasks on Iran's behalf. Both initiatives would allow Iran to pursue legitimate civilian nuclear energy while reducing the risk that the same technology could be diverted to make weapons.

Russian Foreign Minister Sergey Lavrov said it is essential for Iran to quit its ambitions for a while until "a resolution of the problems remaining over the Iranian nuclear program" could be reached.

He said Russia — a longtime ally of Iran — was working to ensure that Tehran maintains its freeze on enriching uranium until talks can be held between Moscow and Tehran over the proposal to move enrichment to Russia.

Negotiations aimed at getting Iran to give up its nuclear aspirations by the EU-3 have been stalled for months as Iran has insisted it has a right to nuclear energy and does not seek weapons. The EU-3 members have all issued strong rebukes against Iran for its announcement that it would resume its nuclear research and development.

French Foreign Minister Philippe Douste-Blazy called on Iran on Monday to immediately retract its decision to restart nuclear activities. He said the EU-3 would meet on the issue soon.

Douste-Blazy also called Iran's intention to restart nuclear activities linked to uranium enrichment "reason for very serious concern."

"We call on Iran to go back on its decision without delay and without conditions," he said.

German Foreign Minister Frank-Walter Steinmeier said earlier Monday that Iran was sending "very, very disastrous signals" on its nuclear program that "cannot remain without consequences for the EU-3's negotiation process."

Javier Solana, the European Union foreign and security affairs chief, told Iran on Saturday that if it resumes its uranium enrichment program, it might doom any further negotiations with the 25-nation bloc about economic aid and other issues.

During President Bush's trips to Europe last year he sought, and aides say received, assurances from the EU-3 that if talks failed, they would support a U.N. Security Council referral for sanctions.

The U.N.'s top nuclear watchdog at the IAEA told Sky News last week that he is losing his patience with Iran. Mohamed ElBaradei makes his next report in March, and administration officials say by that time it will be clear if Russia and China will support sanctions.

FOX News' Carl Cameron and Teri Schultz and The Associated Press contributed to this report.