President Bush has decided to back European allies in their plan to offer economic incentives to persuade Iran to abandon any effort to build nuclear weapons, a sharp shift in policy for a government that had long refused to bargain for Tehran's cooperation, senior administration officials said yesterday.

Secretary of State Condoleezza Rice plans to announce the decision as early as today, culminating an intense negotiation over recent weeks that brought U.S. and European leaders together in their approach to Iran after a long split. By agreeing to try incentives first, U.S. officials believe they will later gain European support for taking the matter to the U.N. Security Council if talks fail.

Rice hinted at the decision yesterday before traveling to Mexico. "I think we're really coming to a common view of how to proceed," she said of her discussions with the Europeans, who have taken the lead in negotiating with Iran. "We're looking for ways to more actively support that diplomacy. But I want to be very clear that this is really not an issue of what people should be giving to Iran. This is an issue of . . . keeping the spotlight on Iran, which ought to be living up to its international obligations."

Rice said Iran would have to commit to not using its civilian nuclear power program as a cover for secret weapons development and would have to submit to intensive inspections by the International Atomic Energy Agency. She did not discuss particular incentives, but those on the table include accelerating membership for Iran in the World Trade Organization (WTO) and permitting Tehran to purchase badly needed spare parts for its aging passenger jets.

Deputy Secretary of State Robert B. Zoellick met with British, French and German officials in Washington on Tuesday to work through the details, according to a European diplomat. The two sides "share a common understanding of where our red lines are . . . and when we'd go to the Security Council," the diplomat said. Among the bigger hurdles has been coming up with terms that would win support from the rest of the European Union.

The decision on incentives would put the United States in the position of engaging Iran diplomatically after a quarter-century of hostility. It could also go a long way toward reconciling the country with its traditional European allies, particularly France and Germany, which broke with Washington over the war in Iraq. Bush's recent trip to Europe, including meetings with French President Jacques Chirac and German Chancellor Gerhard Schroeder, broke through a long logjam over Iran.

But it was not at all clear that incentives would be enough to persuade Iran, which denies trying to build nuclear weapons, to give anything up.

"It's a nose in the tent, but the concessions themselves were really disparaged by Iranians that I spoke with as not meaningful," said Clifford Kupchan of the Nixon Center, a think tank. "In reality, it would take years for Iran to accede to the WTO, and airplane parts, while badly needed, just aren't of the scale that would induce Iran to begin to consider trading their crown jewels."

Still, Kupchan, who just returned from a two-week trip to Tehran during which he met with senior Iranian officials, said U.S. involvement in the negotiations is essential to Tehran. "It's clear that the Iranians view U.S. participation in the E.U. talks as critically important," he said. "It's also clear that they view these first steps as insufficient to engage them in meaningful discussions on the nuclear issue."

Hadi Semati, a visiting public policy scholar at the Woodrow Wilson International Center for Scholars and a University of Tehran political scientist, agreed that Iranian leaders would dismiss the incentives as inadequate.

"They want much more serious carrots, much more serious discussions of security guarantees," he said. "I don't think it will impress the Iranians at this stage, given the backdrop of rhetoric warfare by the administration over the past couple of months." More than airplane parts, the Iranians are looking for a broad change in U.S. strategy toward Tehran, he said. "They want a new paradigm of rapprochement to Iran. That's the price of giving up any program."

Bush's willingness to go along with incentives of any kind stems from a desire to gather support for later punitive action, assuming the incentives do not work, and to present a united front before the Security Council. Rice plans to disclose the incentives decision in an interview with the Reuters news agency today, according to officials who spoke on the condition of anonymity.

Staff writer Glenn Kessler contributed to this report.