The standoff between Iran (search) and Europe escalated into a showdown Wednesday, with Tehran poised formally to end its freeze of activities that can be part of the process of making nuclear weapons — a move that could lead to action by the U.N. Security Council.

Reached by telephone, senior Iranian envoy Sirous Nasseri (search) confirmed to The Associated Press that he had flown just hours earlier to Vienna with a letter from his government to the International Atomic Energy agency, the U.N. nuclear watchdog charged with overseeing Tehran's suspension of uranium enrichment and related activities.

Nasseri declined to discuss the contents of the letter. But diplomats close to the IAEA (search) said they expected it contained formal notification that Iran was resuming conversion of uranium ore as part of a process whose end result is uranium hexafluoride — a substance that then can be used to produce weapons-grade uranium.

One of the diplomats — who like his colleagues spoke on condition of anonymity because of the sensitivity of the issue — said he expected Nasseri to present his letter Thursday.

Any such formal notification of resumption of conversion will torpedo Iran's talks with France, Germany and Britain. Those talks are intended to lessen suspicions about Tehran's ultimate nuclear aims. The United States says Iran wants to make the bomb, but Iran insists it is interested in the atom only as a source of energy.

Washington has long maintained that Iran's nuclear program — kept secret for nearly two decades until revealed by a dissident group in 2002 — is meant to make weapons, and as such, Tehran's nuclear dossier belongs in the hands of the Security Council. But because of strong resistance at previous IAEA board meetings, it reluctantly embraced the European diplomatic efforts.

The on-off talks, which began last year, have failed from the beginning to find common ground on the European insistence that Iran scrap — or at last agree to a long-term suspension of — uranium enrichment and related activities, and Tehran's insistence that any freeze was voluntary and short-lived. The last formal round ended inconclusively April 29.

The Europeans appeared braced for the inevitable. Diplomats on Wednesday said the three nations had begun informal contacts with the IAEA about convening a special session of its 35-nation board should the Iranians tell the agency they were ready to break IAEA seals on conversion equipment in the central city of Isfahan.

Such a session could be called within days of formal notification by Iran of plans to resume conversion. The diplomats said a likely scenario would see board nations giving Iran a two- to three-week deadline to change its mind. If it refused, sentiment at the next board meeting — probably in June — would be strong to declare Tehran in violation of its agreements to suspend enrichment while negotiating in good faith with the Europeans. In that case, the board might refer the case to the U.N. Security Council.

One senior Western diplomat said the three European nations also were consulting with the United States on a common course of action. A senior U.S. official in Washington said earlier that the Bush administration was conferring closely with the allies and that all the governments were determined there would be consequences for Iran if it ends the moratorium.

While the Europeans had been key in previous board opposition to referring Iran to the Security Council, senior officials in several European capitals suggested any resumption of reprocessing would leave them no choice but to support such a move.

"I think the reaction of the ... Europeans is going to be very tough if conversion resumes," said one. "It's not possible to get the Europeans scared."

Suggesting that European patience had run out with Iran's negotiating tactics, another said: "The game of poker is over." Both spoke on condition of anonymity.

A third official said the three European nations had delivered a strongly worded letter to the Iranian government formally warning it against resuming any enrichment-related activity.

In another sign that conversion was about to start again, Iranian Vice President Gholamreza Aghazadeh said Tuesday that reprocessing could resume as soon as Thursday. He said the move is a message to Europeans that Iran can't continue offering unilateral concessions against nothing from Europe.