The White House yesterday endorsed a proposal that would allow Iran to refine uranium at a key nuclear facility as long as more advanced work on the material was completed in Russia, reversing a major element of Bush administration policy in the hopes of resolving a crisis over Tehran's nuclear ambitions.

The Russian proposal, delivered to Iran last weekend, is designed to eliminate any chance that Iran could enrich uranium for weapons. Instead of being enriched at home, the converted uranium, known as UF6, would be enriched in Russia and then sent back to Iran to fuel a Russian-built nuclear power reactor there.

"We hope that over time Iran will see the virtue of this approach, and it may provide a way out" of the crisis, President Bush's national security adviser, Stephen J. Hadley, said in South Korea after the president met with Russian President Vladimir Putin at a summit there. Until yesterday, the administration had insisted publicly that Iran abandon all aspects of its nuclear program. For the first time, the White House also appeared to accept Iran's legal right to enrich uranium , as long as it did not act on it.

Iran did not immediately react to the U.S. endorsement of the proposal. Its officials initially rejected the Russian offer but then made cool public statements saying they would consider it. The Islamic republic took out a full-page ad in the New York Times yesterday defending its nuclear energy program and calling for a negotiated resolution to the crisis.

Officials involved in crafting the new U.S. approach said it signals a growing recognition inside the Bush administration that its Iran policy, both tactically and strategically, was failing to resolve a two-year crisis over the country's nuclear program.

No deal, with terms acceptable to Washington, has been reachable between Iran and other parties in the talks, including European nations. At the same time, the Bush administration has been unable to persuade allies to send the Iranian nuclear case to the U.N. Security Council, where the country could face economic sanctions.

"This is a genuine move on a substantive element of our policy. We have been convinced that if the deal goes through right, then it won't constitute a proliferation risk," said one senior official who would discuss internal deliberations only on the condition of anonymity. If Iran rejects the deal, the official said, it will be easier to "build a coalition to increase pressure."

Iran has said its program was designed to produce energy, not bombs, and that it has a right to a peaceful program under the nuclear Non-Proliferation Treaty. But the scale and clandestine nature of the program, built in secret over 18 years, have fed suspicions that it is a cover for bomb-making.

The 35-member board of the International Atomic Energy Agency meets in Vienna on Thursday to discuss the status of Iran's program, and U.S. officials said they hope the Russia deal can avert another round of diplomatic difficulties over how to handle the situation. Although many countries appear to share U.S. suspicions about Iran's intentions, they have profound differences over how to respond.

Yesterday, IAEA inspectors in Vienna revealed that Iran has possessed, at least since the mid-1980s, the technical know-how to manipulate uranium for the core of an implosion device. The find, contained among a trove of documents Iran turned over to inspectors in recent weeks, could alter the course of the three-year investigation that so far has not turned up proof of a weapons program in Iran. The inspectors' report said Iran obtained the document from a black-market nuclear network operated by Pakistani scientist Abdul Qadeer Khan.

Khan, who also supplied Libya and North Korea, made an initial offer to Iran in 1987 for a nuclear weapons starter kit. The newly disclosed documents appear to have been the result of that offer. Iranian officials have said they agreed only to buy equipment that could be used for an energy program and rejected those items specifically offered as part of a weapons program.

But the existence of the documents opened a new line of inquiry for inspectors.

"There is still a lot more to investigate here," said Charles D. Ferguson, a nuclear specialist at the Council on Foreign Relations. "The big question is what the Iranians did with the documents. Did they act on this or shelve it away?"

Iran has said it did not receive a warhead design from Khan, as Libya had. U.S. intelligence secretly obtained schematic drawings related to Iran's missile program last year that show efforts to modify a ballistic missile to carry a payload large enough to accommodate a nuclear warhead. But the drawings do not include warhead designs, U.S. officials have said.

Without proof of a weapons program, the Bush administration's main focus has been on preventing Iran from enriching uranium because the process can produce fuel for energy or bombs. Currently Iran is converting uranium -- a process that readies the material for enrichment.

"They have to eliminate the possibility that they'll be engaged in any process, nuclear energy fuel cycle process," R. Nicolas Burns, the undersecretary of state for political affairs, said two months ago. "Our position is they should shut down all those activities, including uranium conversion."

The Russian deal endorsed by the U.S. administration offers Iran the chance to continue converting uranium, and to retain the right to enrich it, just not on its own soil.

Hadley said it was an interesting idea and he hopes Iran will embrace it: Iran, "while retaining its right to enrichment and reprocessing, would, nonetheless, find it in its interest to give up that right in terms of its own territory."

Robert Einhorn, assistant secretary of state for nonproliferation until November 2001, said the Russian offer takes care of Iranian needs and U.S. concerns. "If you are confident that Iran has no enrichment plant in operation, overtly or covertly, then it doesn't much matter that it can produce" converted uranium.

Staff writer Peter Baker in Pusan, South Korea, contributed to this report.

A technician works at a uranium-processing plant at Isfahan.