The warning by Britain, France and Germany came in response to a statement by a top Iranian nuclear official earlier in the day that Tehran could "momentarily" announce a resumption of "a noticeable part" of uranium conversion work.

In a letter to Tehran, Britain, France and Germany warned that any violation of a November 2004 accord under which Iran froze its sensitive fuel cycle work -- the focus of suspicions of a nuclear weapons drive -- would have "consequences" for the country.

The letter also "proposes a four-way meeting in the near future."

In Brussels, European Union foreign policy chief Javier Solana confirmed that the warning letter had been sent to Iran.

"We communicated very clearly to our Iranian interlocutors that that will have consequences if they decide to breach or to do something that for us is a breach of the agreements of November," he told reporters.

"Let's wait and see what actually happens," British Prime Minister Tony blair told a press conference when quizzed about a response to Iran's threats.

"But we certainly will support referral to the UN Security Council if Iran breaches its undertakings and obligations.

"Quite how that will come about we have got to work out with our colleagues and allies," Blair added. "But those international rules are there for a reason, and they have to be adhered to."

In Paris, French Foreign Minister Michel Barnier took the same line, saying: "We continue to hope that Iran will not take this step, the consequences of which it is well aware."

Any decision to start converting uranium would "be counter to the Paris agreement and resolutions adopted by the International Atomic Energy Agency (IAEA). We want to get this position across to the authorities in Tehran," he said.

"We hope the Iranians will reconsider," an EU diplomat said, adding that Iran was certain to be referred to the Security Council if it did not.

Britain, Germany and France have offered Iran a package of incentives in return for "objective guarantees" that it will not develop weapons.

But Iran, insisting on its "right" to possess nuclear technology for peaceful purposes, warned that the mounting pressure could undermine its commitment to the Non-Proliferation Treaty (NPT) -- the cornerstone of the global effort to stop the spread of nuclear weapons.

"We believe that a part of our nuclear activities must be restarted, but we are discussing the conditions and timing," top nuclear negotiator Hassan Rowhani told state television.

"But the main point is that we will resume a part of our activities in the near future."

Rowhani was also quoted as telling visiting Russian Deputy Minister of Foreign Affairs Sergei Kislyak, "If Iran cannot exercise its rights with in the framework of the NPT, it will no longer have any respect for this treaty."

"The Iranian people," he asserted, "will not accept giving in to force and are prepared to pay any price.

"Iran is not looking for a nuclear bomb, it wants to cooperate so that there are no international worries," he repeated.

In Washington, White House spokesman Scott McClellan reiterated that the United States backed Europe's approach toward Iran and urges Tehran to adhere to its international obligations.

"We continue to support the efforts by the Europeans to resolve this matter and to make sure that there is an objective guarantee in place that Iran is not developing nuclear weapons under the cover of a civilian program," he told reporters.

"Iran needs to abide by its international obligations," McClellan said.

Washington has been skeptical of European efforts to talk Iran out of its nuclear ambitions but has actively backed the initiative since March.

Solana said he did not want to be overly dramatic but warned that if Iran restarts enrichment-related activities "the first thing we would like to do is call a meeting in Vienna to analyse the situation."

Diplomats at the IAEA in Vienna said last-minute talks were underway with Iran, which earlier appeared poised to officially inform the nuclear watchdog of its intention to break UN seals and resume uranium conversion -- a precursor to the highly sensitive enrichment process.

Iran has voiced frustration with the pace of the negotiations, which remain deadlocked over Iran's ambition to master the full nuclear fuel cycle and European demands that Iran abandon such work altogether.

It maintains that it need nuclear power to meet increased energy demands from a booming population and to free up its vast oil and gas resources for export and thus earn badly-needed hard currency.

The standoff has worsened after the three EU interlocutors last month rejected an Iranian proposal to begin a phased resumption of enrichment. Iran complained the talks were being deliberately dragged out so as to keep the freeze in place.

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