VIENNA -- The head of the International Atomic Energy Agency said Wednesday that Iranian negotiators had agreed to a draft of an agreement to ship much of its stockpile of nuclear fuel to Russia, but cautioned that it would have to be approved by Friday in both Tehran and Washington.

The draft, which came after three days of talks here between Iran, the United States, France and Russia, fills in the details of an agreement in principle made on Oct. 1 following a preliminary round of negotiations.

If approved, the deal would commit Iran to temporarily exporting 75 percent of its known stockpile of low-grade nuclear fuel to Russia, or about 2,600 pounds of low-enriched uranium, for additional enrichment. Negotiators say that would prevent the possibility that Iran could turn the fuel into weapons-grade material anytime soon.

Western suspicions that Iran is secretly developing a nuclear weapon despite its repeated denials are at the heart of the negotiations, which represent the first time the United States and Iran are in serious diplomatic talks after 30 years of estrangement. The talks are regarded as an important test of the Obama administration's policy of seeking to engage America's adversaries.

But the key to the agreement reached in the talks, if it works, would be in the timing of the shipments -- a detail officials were not discussing in Vienna in the hours after the announcement. If Iran actually sends the low-enriched uranium to Russia in a single shipment, as the draft document states, it would have too little fuel on hand to build a nuclear weapon for roughly a year, according to the agency's experts. If the fuel leaves Iran in batches, the experts warn, Iran would have the ability to replace it almost as quickly as it leaves the country.

Also of concern is the possibility that Iran might have more nuclear fuel than it is letting on. The estimate that Iran has about 3,500 pounds of low-enriched uranium assumes, as one senior European diplomat put it on the sidelines of negotiations here, "that Iran has accurately declared how much fuel it possesses, and does not have a secret supply."

Time is critical to President Obama because he is seeking the space to make a broader deal with Iran, and the ability to hold off Israel, which has hinted that it could take military action next year if it believed Iran was getting close to the ability to produce a nuclear weapon.

With the clock ticking, Mr. Obama is under pressure to show that there have been early fruits from the decision to engage directly with Iran on the status of its nuclear program. Mr. Obama made that promise during the presidential campaign, after President Bush declined to enter substantive talks with Iran while it was continuing to enrich uranium.

Ultimately, Mr. Obama would have to get Iran to agree to give up the enrichment process as well; otherwise, the fuel taken out of circulation in the draft agreement would soon be replaced. During the campaign, Mr. Obama and his aides said that Iran could not be trusted to enrich uranium. But he has not made the cessation of enrichment a prerequisite to talks, and the work is still under way, in violation of three United Nations Security Council resolutions.

It was not immediately clear why a final agreement could not be declared, but it appeared that the Iranian delegation did not have the authority to sign such a critical agreement on which the leadership in Tehran appears divided. Mohamed ElBaradei, the director general of the I.A.E.A., said that he hopes leaders "see the big picture" and approve the agreement by the end of the week.

Iran's representative to the I.A.E.A., Ali Asghar Soltanieh, told reporters on Wednesday that while his team of negotiators had accepted the draft agreement, senior officials in Tehran would have to approve it. "We have to thoroughly study this text," he said.

American officials said nothing about the deal in the hour after it was announced by Mr. ElBaradei, who had told his staff that "failure is not an option" in pressing for an agreement. Mr. ElBaradei said that before he leaves office at the end of November, the West must be assured that Iran has given up what in nuclear parlance is called "breakout capacity." That is the ability to renounce the Nuclear Nonproliferation Treaty and try to convert reactor fuel into bomb fuel.

The 2,600 pounds of fuel would be further enriched in Russia, and ultimately shipped back to Tehran for use in an aging reactor that serves medical purposes. But it would be converted into a form that would be difficult to use in a weapon.

The United States and the European Union insisted on Oct. 1 that Iran had agreed to the outlines of this agreement, but Iranian diplomats in recent weeks cast that into doubt, reminding world leaders that they had no intention of giving up their ability to enrich nuclear fuel. Calculations by the International Atomic Energy Agency and outside researchers concluded that it would probably take Iran about a year to replace the 2,600 pounds of fuel with new production, which it is carrying out in violation of three United Nations Security Council resolutions.

First Published: October 21, 2009, 2:30 p.m.