copyright 2009, The Washington Post WASHINGTON -- Iran on Thursday appeared to reject a key element of a U.N.-backed proposal aimed at quickly reducing its stockpile of enriched uranium, offering an informal, oral counter-offer that diplomats said fell far short of a tentative deal reached earlier this month. Mohamed ElBaradei, director general of the International Atomic Energy Agency (IAEA), told the Iranian ambassador to the U.N. agency that the Iranian counter-offer, as structured, would not be acceptable to Russia, France and the United States -- the other parties to the arrangement -- and urged him to get more clarification from his government. Diplomats said they hope a formal, written answer from Iran will be delivered as early as Friday. The long-awaited Iranian answer appeared to dash hopes that Iran would be willing to quickly embrace engagement with the West on its nuclear program. Not only did Tehran appear to reject a central element of the proposed agreement, it also has refused to commit to another high-level diplomatic meeting to discuss the program. Obama administration officials will now need to assess whether the engagement gambit has begun to run its course -- and whether to shift toward pressing for tougher sanctions against the Islamic Republic. In a statement, the IAEA said that ElBaradei 'has received an initial response from the Iranian authorities' and that he 'is engaged in consultations with the government of Iran as well as all relevant parties, with the hope that agreement on his proposal can be reached soon.' The agency provided no other details. In talks in Geneva on Oct. 1, Iran tentatively agreed to the arrangement, under which nearly 80 percent of its stockpile would go to Russia and France to be fashioned into fuel for a research reactor that produces isotopes that detect and treat diseases. As part of the deal, the United States would support the IAEA in an effort to help Iran ensure the safe operation of the aging reactor, first built by the United States in the 1960s. Iran has enough low-enriched uranium, in theory, to produce one nuclear weapon. If it agreed to the deal, most analysts estimate, it would be nine to 12 months before Iran would again have enough uranium to be able to enrich it to weapons grade. Further talks were held last week in Vienna, with ElBaradei presenting a draft agreement that was embraced by the other countries, but Iran missed a Friday deadline to respond. A central element of the plan, first conceived by the Obama administration, is that Iran must ship the enriched uranium out of the country in one batch by the end of the year. Instead, the presentation by Iranian Ambassador Ali Asghar Soltanieh suggested that Iran would ship out its enriched uranium in batches, swapping it for new material on a continuous basis, diplomats said. That would negate the main attraction of the proposal to the major powers dealing with Iran, because it would mean Iran's stockpile of enriched uranium would not be significantly reduced. The United States, France and Russia had no official response to the Iranian counter-offer, but they were consulting behind the scenes about how to respond if the formal offer differed little from the ambassador's apparent trial balloon. One U.S. official, speaking on the condition of anonymity, described Iran's answers as 'a response of sorts' but said the other three countries remain united in support of the plan. 'We need further clarification,' State Department spokesman Ian Kelly told reporters. 'And I think it's also fair to say that we need to have a formal response from Iran at this point. We've been given some details of it, but we're still talking to the Iranians about it.' 