Tehran, suspected by arch-enemy the United States of seeking nuclear weapons, said it would restart uranium conversion activities when inspectors from the International Atomic Energy Agency arrived at its Isfahan plant.

"If the Europeans call an extraordinary meeting (of the IAEA board), that will be a violation of all international rules and they should not expect us to maintain the freeze on our activities at the Natanz enrichment plant," Hossein Moussavian, spokesman for the nuclear negotiating team, told state television.

But European diplomats responded by saying they had asked for an emergency meeting of the IAEA board next Tuesday, with the intention of pressuring Tehran into backing down again.

"The instruction has been sent" to the IAEA, a diplomat from one of the three European Union countries negotiating with Iran told AFP in Vienna.

The IAEA, whose inspectors are currently in Iran on a routine mission, says it would take a week for its team to put in place the necessary checks to monitor Iran's resumption of the uranium conversion.

"The safeguards equipment will not be in place until the middle of next week," IAEA spokeswoman Melissa Fleming said.

Iran agreed in November to suspend its enrichment and conversion work -- part of the cycle that can make fuel for use in a nuclear bomb -- while negotiations with the Europeans are going on.

But in recent days it has threatened to resume conversion work, triggering warnings from the Europeans that they would halt negotiations and may refer Iran to the UN Security Council for possible sanctions.

The European Union is expected to present Iran with a set of trade and technology incentives in exchange for a guarantee from the country that its nuclear programme is peaceful.

Iran said it had planned to resume the fuel-cycle work on Wednesday -- the day ultra-conservative President Mahmood Ahmadinejad took office -- but later signalled it would delay the start, in effect giving more time for diplomacy.

The EU's representatives Britain, France and Germany hoped for more time to finalize the package of incentives.

A State Department official said the so-called EU-3 had informed Washington they would unveil the package over the weekend, perhaps as early as Friday. He gave no other details.

But Iran has said it remains determined to resume its uranium conversion work at the Isfahan plant. The process is the first step in the cycle to produce fuel for nuclear reactors.

In a letter sent to the foreign ministers of the EU-3 on Tuesday, chief nuclear negotiator Hassan Rowhani said this "inalienable right cannot be the subject of arbitrary and subjective criteria".

"Your side has continued to refrain from responding substantively to our proposals, in whole or even in part. We intend to take a minimal step," the letter said.

"The Islamic Republic of Iran stringently desires a negotiated resumption of its enrichment activities and is prepared to continue in good faith and in an expeditious and result-oriented manner its negotiations," it said.

Ahmadinejad, who ever since his shock June election victory has pledged to lead a government of moderation, has eschewed making any explicit comment on the nuclear programme and confined himself to vaguer rhetoric.

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