The Brussels talks aim to prepare the way for negotiations in Geneva on Wednesday led on the European Union-side by the trio's foreign ministers plus EU foreign policy chief Javier Solana.

"Nobody wants a crisis on our side. We want the talks to continue," said one EU diplomat ahead of the closed-door meetings.

The so-called EU-3, representing the full 25-member EU, called the talks after a series of recent threats from Tehran to resume key nuclear activities, in breach of an accord to suspend them last November.

The EU, in contrast to the United States which suspects Tehran of wanting to build nuclear bombs, is seeking to engage the Islamic state, using the carrot of possible trade and other benefits to persuade it to curb its nuclear plans.

But at the same time it has warned Tehran starkly that it could be referred to the UN Security Council -- and into Washington's diplomatic line of fire -- if the talks with the Europeans break down.

"Iran should be in no doubt that any such change to the suspension would be a clear breach of the Paris agreement" of last November, the EU-3 said in a letter to Iran's top negotiator Hassan Rowhani, calling for the talks.

"It would bring the negotiating process to an end. The consequences beyond could only be negative for Iran," added the letter, a copy of which was obtained by AFP.

Iran has warned bluntly that the talks are the "last chance" for the Europeans to offer it enough of an incentive to stop it resuming uranium enrichment activities, as threatened.

Ali Agha Mohammadi, spokesman for Iran's Supreme National Security Council, said Monday that the Iranians may not even travel to Geneva if no satisfactory offer is made during talks between experts in Brussels.

"We would reach the conclusion that we haven't got along with them," if they fail, he said.

Speaking on the eve of the talks, British Foreign Secretary Jack Straw acknowledged that the meetings in Brussels and Geneva will be difficult -- but said he remains optimistic.

"The issue before us will be to ensure that both sides stick by the agreements which we have already entered into," said Straw. "I think (the talks) will be tough, but I think very much they will be successful."

Tension with the EU was further fueled on the eve of the talks by the disqualification by Iran's Guardian Council of most pro-reform candidates for June presidential polls.

The Council, an unelected watchdog body that vets all candidates for public office, announced on Sunday that just six men out of 1,014 would-be candidates can stand to succeed incumbent reformist President Mohammad Khatami.

"We were very disappointed by the decision of the Guardians Council," said Luxembourg Foreign Minister Jean Asselborn, whose country currently holds the EU's rotating presidency.

The venue for the Brussels talks was being kept closely under-wraps, but one official told AFP they were expected to start mid-morning (0830 GMT) in one of the EU-3's diplomatic missions.

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