GENEVA -- On the eve of crucial negotiations over Iran's controversial nuclear program, U.S. and Iranian diplomats expressed hope Wednesday that they will be able reach an initial understanding leading to a comprehensive accord by Friday.
The deal would involve lifting for six months some of the international economic sanctions that have been imposed on Iran in exchange for its curbing the enrichment of uranium.
Such a deal would mark a breakthrough in the decade-long dispute over a program that the United States, Britain, France, Germany, Russia and China fear is intended to produce a nuclear weapon.
"What we're looking for now is a first phase, a first step, an initial understanding that stops Iran's nuclear program for the first time in decades and potentially rolls some of it back," a senior U.S. official said. The official, a member of the U.S. delegation led by Undersecretary of State Wendy Sherman, couldn't be identified under the rules of the briefing.
At the heart of the proposal is the demand that Iran halt expansion of its ability to enrich uranium, presumably by not buying new centrifuges, the equipment used in the enrichment process. That is a change from previous demands that Iran stop enriching uranium past a certain purity.
Iran already has produced a sizable stockpile of low-enriched uranium and a more worrisome supply of 200 kilograms -- 440 pounds -- of uranium enriched to 25 percent purity. Experts say there is no clear need in Iran's peaceful nuclear program for the latter amount, and that if Iran decided to, that 25 percent could be enriched quickly to the high-level purity needed to produce a nuclear weapon.
Iran's new foreign minister, Javad Zarif, said Iran was "prepared to reach an agreement" in the talks this week.
"We are optimistic that we can move forward," he told France 24, an all-news television channel. Mr. Zarif said that while the negotiations were expected to take at least a year to complete, Iran needed to have a sense of the final outcome.
He appeared to be referring to Iran's demand to end the most devastating sanctions, which limit oil sales and international banking relations.
The accord under discussion this week, Mr. Zarif said, addresses the most immediate concerns of Iran, the United States and the five other nations. "I believe it's not difficult to reach that agreement," he said. If the six major powers "are prepared to reach an agreement, then we can have an agreement."
Because so little has come out about behind-the-scenes talks since they kicked off in Geneva in mid-October, it wasn't clear whether the two sides were genuinely on the edge of an accord or whether both were posturing and professing eagerness for a deal to increase psychological pressure on each other.
The senior U.S. official said the six powers were willing to expand on an offer they had made in February at talks in Almaty, Kazakhstan, that Iran rejected at the time. That proposal offered to ease bans on sales of Iran's petrochemicals and precious metals in return for a complete halt in uranium enrichment beyond a 20 percent level. This time, it's a sliding scale.
The six-month period for the interim agreement is only a proposal and hasn't been agreed to, the official said.
One of the biggest questions as the talks resume in Geneva is whether the U.S. Congress will impose more unilateral sanctions against Iran, a step Israel and its supporters in Congress favor, but that the Obama administration adamantly opposes.