The United States, Russia, China and key European powers may for the first time be ready to modify demands that Tehran completely dismantle a nuclear program that could produce the fissile material for nuclear warheads, foreign government officials said Tuesday.

Speaking on the eve of talks between top Iranian envoy Ali Larijani and Javier Solana, the European Union's foreign policy chief, the officials — some of them diplomats, others based in their capitals — said the discussions were key because for the first time they could try to sidestep the deadlock over uranium enrichment by trying to agree on a new way of defining it.

Iran now is running more than 1,300 centrifuge machines and — as a prelude to enrichment — has recently coated their insides with minute amounts of the uranium gas that is used for enrichment itself, according to an internal document of the International Atomic Energy Agency, the U.N. nuclear agency.

Iran's ultimate stated goal is to run 50,000 centrifuges a year, enough to churn out material for a network of nuclear power generators — or a full-scale nuclear weapons program, should it choose to do so.

One of the diplomats said recognition by the United States and its allies that Iran would never accept their earlier demand of a full freeze dictated a decision to contemplate "a new definition of enrichment" that would allow Tehran to keep some of its program intact without actually turning out enriched material.

"The prize is the 50,000," he said, alluding to attempts by the six world powers to prevent Iran from developing its full-scale program at its underground enrichment facility at Natanz. He, like other government specialists in nonproliferation and Iran's nuclear program who spoke to The Associated Press, demanded anonymity because their information was confidential.

The United States was favoring "cold standby" — where a set number of centrifuges are allowed to remain standing and assembled in series but not running, said the diplomat. Iran, he said, was likely coming to Wednesday's discussions seeking "hot standby" — where the machines are at least operating, if not producing enriched uranium.

The six powers also wanted to reduce assembled and hooked-up centrifuges to less than 1,000, so — should Larijani and Solana agree that there was further room for discussion — numbers also would likely play a role in any future talks, he said.

At best, however, the two sides on Wednesday could only agree to meet again. Solana needs to report back to the six capitals that he is representing, while Larijani likely needs to have approval of anything discussed at the Ankara talks.

Solana on Monday had spoken of a "possibility of moving the process forward," while Larijani said he hoped for "a serious negotiation towards reaching a solution."

Asked for hopes about Wednesday's exploratory meeting, Solana spokeswoman Cristina Gallach said: "We have to create the conditions for negotiations."

Iran's defiance of a U.N. Security Council demand to freeze all activities linked to enrichment — a possible pathway to nuclear arms — has led to two sanctions-bearing resolutions against Tehran, the latest in March. Although the punishments are selective and relatively mild, they could be further sharpened if the Islamic republic refuses to compromise.

The United States and others say past suspicious nuclear activities — including a program Tehran kept secret for nearly two decades — make Iran a special case.

But Tehran argues the sanctions are illegal, saying that it — like other nations that have endorsed the Nuclear Nonproliferation Treaty — has the right to enrich to generate nuclear power. That, say Iranian officials, is the only purpose of their program, rejecting suspicions that they want ultimately to enrich to weapons-grade uranium for the fissile core of nuclear warheads.

The last face-to-face talks between Solana and Larijani were more than six months ago, and they foundered over the same issue. Solana, representing the United States, Russia, China, Britain, France and Germany, demanded that Iran mothball not only fledging enrichment efforts but all linked aspects, including assembling centrifuges to enrich and facilities to house such plants. Tehran refused.

Earlier negotiations between the three European nations and Iran broke up after Tehran refused to accept a full freeze of all enrichment-related activities in exchange for a package of economic and political inducements, including help in developing a peaceful nuclear program.

The approach on both sides before Wednesday's talks, however, might make a compromise easier, because of a new willingness to examine possible ways of redefining an enrichment freeze, said the officials.

"We purposely left open the possibility that direct talks could happen by being a little less committed to the requirements to have a meeting," said a U.S. official, alluding to previous demands of an all-encompassing freeze on all enrichment related activities.

A European official said hopes were that both sides could agree on at least "a different definition of suspension that we can work with."