The United States yesterday effectively and reluctantly agreed to allow three European nations to launch a final diplomatic initiative aimed at persuading Iran to accept a plan that would block it from developing a nuclear weapon, U.S. and European officials said.

Increasingly alarmed by Iran's potential to develop nuclear weapons, Germany, France and Britain -- endorsed by the Group of Eight, the world's wealthiest alliance -- will meet next week with Iranian officials to make it clear that Tehran faces a choice: Embrace the process outlined in this diplomatic effort or face the possibility of new pressures or punitive action from the United Nations, the officials said. The terms were outlined during talks at the State Department yesterday between the United States and its G-8 allies.

The outline of a two-stage compromise is emerging from the Europeans' overture to Tehran, said several European officials familiar with the plan. During the first stage, effective immediately, Iran would indefinitely suspend all efforts at developing an independent fuel cycle for its nuclear energy program but not give up its right to enrich uranium, the officials said. Enriched uranium can be diverted to military uses.

That first stage would have to be completed and verified by a team from the International Atomic Energy Agency before the U.N. nuclear watchdog agency's next board meeting, in late November.

The second stage would involve broader talks on long-term security and economic issues, to help "normalize" Iran's place in the world and to reassure the international community about various Iranian policies, said a senior European envoy familiar with the plan.

The Europeans, for example, would be willing to explore new trade agreements, technology cooperation and other assistance while working out a permanent agreement on Iran's nuclear program and discussing other contentious issues, such as terrorism and the Middle East peace process.

At some stage of the negotiations, the Europeans expect the United States to participate in the diplomatic effort, ending 25 years of tensions, the European envoy said. The overall goal is to signal to Iran that it should not need to develop its own fuel cycle.

Iran's first nuclear reactor at Bushehr, a 1,000-megawatt facility Russia just completed, is due to start up in 2005 or 2006. The Europeans want an agreement whereby Moscow provides the fuel to Iran, and Iran returns the spent fuel to Russia, the envoys said.

The sweeping potential for cooperation is a "dream offer" that could make Iran "the big winner in the region," said the senior European envoy, who spoke on the condition of anonymity because of the sensitive diplomacy.

The State Department called yesterday's talks "useful." But the U.S. delegation, led by Undersecretary of State John R. Bolton, expressed deep skepticism that Iran will comply, given its failure to follow through on an agreement with Germany, France and Britain a year ago.

"We emphasized to our G-8 partners that Iran should not be allowed to defy any longer the requirements and requests called for in the past five IAEA resolutions," said State Department press officer Edgar Vasquez.

A senior State Department official, speaking anonymously because of the sensitive diplomacy, added that there has been no sign that the Iranians will take this initiative any more seriously. "So, unfortunately, our feeling is that the Iranians are still Iranians," he said.

The G-8 -- which also includes Japan, Canada, Italy and Russia -- was brought in to widen the leverage. "We're not alone anymore," said one envoy from the original three countries party to the talks.

The specifics of the four-page European plan are scheduled to be put forward next week, possibly Thursday in Vienna, European and Iranian envoys said. Other G-8 members will pursue parallel bilateral discussions directly with Tehran, European envoys said.

In an interview, a senior Iranian official said Tehran is ready to take "very positive measures for confidence building," including "voluntary, temporary" steps to ensure that the country will not divert nuclear technology for use in weapons.

For Iran, the most important element in any agreement is recognition of Tehran's right to nuclear technology for energy, he said. "There is a good possibility for a solution. Providing this condition which emanates from our insistence on mutual respect is met, then Iran will be forthcoming," he added on the condition of anonymity because of the ongoing diplomacy.

But he also warned that the Islamic republic does not want to be confronted or faced with "pressure tactics." "The Europeans know our red lines, and the room for maneuver is well understood," he said. "But if they want to set specific demands and use pressure tactics, then it will be dead on arrival."

The initiative emerged not only because the Europeans want to try at least one more time, but also because of the potential difficulties of winning agreement at the IAEA meeting next month and then at the U.N. Security Council if Iran does not comply, European envoys said. Non-aligned countries such as South Africa, Brazil and Malaysia fear that any move against Iran would set a precedent limiting their potential to develop nuclear energy programs, the diplomats said.

"It's one thing to say we'll go to the Security Council," said a European informed about the new diplomacy, "and another thing to get there."

The Bushehr reactor is the focus of a plan by Germany, France and Britain. Russia would provide fuel; Iran would return the spent fuel to Russia.