LAUSANNE, Switzerland (AP) -- Iran may accept new constraints on uranium enrichment but is pushing back about the length of limits on technology it could use to make nuclear arms, Western officials told The Associated Press on Sunday.
Tuesday is the target date in the nuclear talks for a preliminary agreement that would set the stage for a further round of negotiations toward a comprehensive deal by June 30. The goal is a long-term curb on Iran's nuclear activities, with Tehran gaining relief from the burden of economic penalties imposed by the West.
Foreign ministers and other representatives from Iran and the six powers in the talks have said the negotiations have a chance of succeeding by Tuesday.
The two Western officials who outlined the state of the talks spoke on condition of anonymity because the officials were not authorized to discuss them publicly.
White House spokesman Josh Earnest said U.S. negotiators were aiming for a strong deal. By accepting constraints on their atomic activities, the Iranians would "live up to their rhetoric that they are not trying to acquire a nuclear weapon," he said in Washington on ABC's "This Week."
Israeli Prime Minister Benjamin Netanyahu renewed his vocal criticism of what he considers a looming diplomatic victory for Iran, and feared that any deal would not stop Tehran from having the ability to produce nuclear arms.
He said the provisions of the deal being worked on appear to "corroborate all our concerns and then some."
Officials said the sides are advancing on limits to aspects of Iran's uranium enrichment program, which can be used to make the core of a nuclear warhead.
Over the past weeks, Iran has moved from demanding it be allowed to keep 10,000 centrifuges enriching uranium, to agreeing to 6,000. The officials said Iran now may be ready to accept even less.
Tehran also is ready to ship out all the enriched uranium it produces to Russia, which the officials said was a change from previous demands that it be allowed to keep a small amount in stock.
One official cautioned that Iran had previously agreed to this but changed its mind.
Iran's official IRNA news agency later cited an unidentified Iranian negotiator as denying his country was ready to move all enriched uranium to Russia.
Uranium enrichment has been the chief concern in over more than a decade of international attempts to cap Iran's nuclear programs.
Tehran says it wants to enrich only for energy, science, industry and medicine. But many nations fear Iran could use the technology to make weapons-grade uranium.
The United States and its allies are seeking a deal that stretches the time Iran would need to make a nuclear weapon from the present two months to three months to at least a year.
The officials said differences on the length of an agreement remain one of the main disputes.
They said Iran wants a total lifting of all caps on its activities after 10 years, whereas the U.S. and others at the talks -- Russia, China, Britain, France and Germany -- insist on progressive removal after a decade.
One official said the two sides may give differing interpretations of any deal -- the Iranians insisting that they are free to do what they want after 10 years, the others listing areas where restrictions remain.
A senior U.S. official characterized the issue as lack of agreement on what happens in years 11 to 15. That official demanded anonymity in line with State Department briefing rules.
Reflecting the tenuous state of talks, the official said no decision had been made on how and in what detail any preliminary deal may be communicated to the public. The real work to be done would begin after Tuesday, the official said, as the sides focus on meeting the June deadline for a comprehensive agreement.
Limits on Iran's research and development of centrifuges also remain unresolved, according to the Western officials.
Tehran has created a prototype centrifuge that it says enriches uranium 16 times faster than its present mainstay model. The U.S. and its partners want to constrain research on such and other advance models, because it would greatly increase the speed that Tehran could make enough weapons-grade uranium for a bomb, once limits on its programs are lifted.
One official said Russia remains opposed to American insistence that any U.N. penalties lifted in the course of a deal be quickly reimposed in case Tehran reneges on any commitments, saying Moscow fears establishing a precedent.
Both officials said monitoring remains a problem, with Iran resisting attempts to make inspections and other ways to make sure there is no cheating as intrusive as possible.
There is tentative agreement on turning a nearly-finished reactor into a model that gives off less plutonium waste than originally envisaged. Plutonium, like enriched uranium, is a path to nuclear weapons.
Iran and the U.S. are discussing repurposing an underground bunker Iran used to enrich uranium to let Iran run centrifuges there. Instead of enriching uranium, the machines would produce isotopes for peaceful applications, they said.
With the deadline close and problems remaining, U.S. Secretary of State John Kerry canceled plans Sunday to return to the United States for an event honoring his late Senate colleague Edward Kennedy.
German Foreign Minister Frank-Walter Steinmeier and Laurent Fabius, his French counterpart, also called off planned trips.
Kerry has been in discussions with Iranian Foreign Minister Mohammad Javad Zarif in the Swiss town of Lausanne since Thursday. The foreign ministers of Britain, China and Russia were to arrive late Sunday.