Iran agreed yesterday to immediately suspend its nuclear programs in exchange for European guarantees that it will not face the prospect of U.N. Security Council sanctions as long as their agreement holds.

The nuclear deal, accepted by Iranian officials in a meeting in Tehran with French, German and British ambassadors, set the stage for a serious test of whether diplomatic engagement is capable of halting Tehran's nuclear ambitions in the long term.

European officials were reviewing Iran's acceptance letter, diplomats said, and expected to brief Washington today before making an official announcement.

The European deal will require months, and possibly years, of further negotiations before Iran agrees to permanently end its nuclear work and falls far short of the strategic decision the Bush administration said Tehran needs to make to convince the world it is not a danger.

For nearly two years, the administration's Iran policy has been based on the threat of taking the Islamic republic to the Security Council. During that time, the White House has remained steadfast in its opposition to negotiating with Iran and has chided European allies for offering inducements to a country President Bush once said was in an "axis of evil" with North Korea and Iraq.

But Washington's closest allies chose another path. A previous agreement reached by the European trio and Iran last year fell apart after six months, leading the Bush administration to argue that negotiations with Tehran produce only empty promises.

Washington's push for Security Council action is unlikely to succeed as long as Iran and the Europeans continue to work together. Officials said that they expect the deal to be lauded in a report today by Mohamed ElBaradei, the director general of the International Atomic Energy Agency, and that the agreement is likely to win the support of much of the agency's board when it meets to discuss Iran on Nov. 25 in Vienna.

U.S. officials, who would discuss the next steps only on the condition of anonymity, said Bush had not decided whether to support the European deal or to lobby the IAEA board to get tougher with Iran. Previous U.S. attempts to win over the board have been unsuccessful.

"That's a decision that will have to be made this week," said one U.S. official involved in Iran policy but unauthorized to discuss it publicly. "But I can't imagine how anyone could argue to the president the tactical benefits of trying to do that again because the result would be U.S. diplomatic isolation."

Senior Bush administration officials have declined to say whether they would seek economic sanctions or an oil embargo against Iran if it was taken to the U.N. Security Council. The public response to those questions has been only: "No options are off the table." But the ambiguity convinced Iran and much of Europe that the White House is trying to take Tehran down a path similar to the one Iraq experienced in the Security Council for more than a decade.

Of the 35 countries on the IAEA board, only Canada and Australia had shown a willingness to refer Iran's file to U.N. headquarters in New York. "That leaves us 2 to 32, and I'm not sure Canada and Australia would even be with us now that there's a deal," the U.S. official said.

Diplomats who have seen drafts of ElBaradei's report said it will be similar to a positive assessment of Iran's cooperation that he offered the IAEA board in September. Several outstanding issues regarding Iran's program remain unanswered, and U.S. diplomats said they would focus on those and pursue a toughly worded resolution against Iran that included more aggressive IAEA inspections and an automatic referral to the Security Council if Tehran breaks any part of the European deal.

Over 18 years, Iran secretly assembled uranium enrichment and conversion facilities that could be used for a nuclear energy program or for the construction of an atomic bomb. The underground sites became a target of a massive IAEA investigation after they were exposed by an Iranian exile group two years ago.

Although Iran's work does not violate international law and U.N. nuclear inspectors have not found evidence that the country is trying to build a bomb, the scale and history of the program have continued to fuel U.S. and Israeli suspicions that Tehran has a covert weapons program. The deal with Europe's three main powers is meant to blunt those suspicions by putting an end to nuclear energy work that could be diverted for bomb making.

The agreement, set out in two phases, began immediately yesterday when Iran informed ElBaradei it had agreed to a full suspension and invited inspectors to verify the commitment.

Western diplomats in Vienna, where the IAEA is based, said that a small team of nuclear inspectors would begin sealing and tagging Iran's nuclear facilities and equipment immediately and that the work would be completed by the time the agency's board meets on Thanksgiving day.

Hassan Rohani, Iran's chief nuclear negotiator, told reporters in Tehran the suspension would remain as long as talks continued on a resolution of Iran's nuclear case. Those negotiations could begin next month, and diplomats said they were discussing a kickoff ceremony to be attended by Iranian, British, German and French foreign ministers.

The four countries involved in the talks will then set up three working groups: one to tackle nuclear issues, another for nonnuclear cooperation between Iran and Europe, and a third for regional security issues. The groups will report every three months to a steering committee made up of senior participants.

Iran had wanted that phase of negotiation to last no more than six months, but Europe insisted that it be open-ended to avoid time-pressured negotiations. One European diplomat said Europe expected the negotiations to last two years or more.

As part of the understanding reached by the parties yesterday, negotiators will spend that time working out a package deal that gives Iran lucrative trade agreements with Europe once it agrees to put a permanent end to its nuclear programs.

The suspension arrangement, which was reached in two days of talks in Paris that ended Nov. 7, nearly collapsed last week after Iran pushed for two changes: an exemption on an early step in the uranium conversion process and promises that the IAEA would close its Iran file. European officials balked at both, and Iran agreed to accept the deal as it had been offered. But the disagreements over interpretations and expectations indicated that future negotiations would not be easy.

European diplomats said Bush's reelection helped the negotiations by limiting Iran's options. Had Democrat John F. Kerry won, Iran might have tried to play for time or probe what policy shifts a new administration was considering, they said.

Staff writer Robin Wright contributed to this report.

Iran's chief nuclear negotiator, Hassan Rohani, says talks will continue on a final resolution.