– Three months into international negotiations over Iran's disputed nuclear development program, Tehran's team has surprised almost everybody with its apparent eagerness for a deal.

Iranian negotiators have met all of their commitments under last November's interim agreement, have proposed compromises on some key disagreements, and have taken part in three top-level meetings without the squabbles that were common over the past decade of fruitless haggling.

Yet President Hassan Rowhani's government is moving away from the U.S. and its allies on an issue that may be the most important of all.

Put simply, the six world powers want Iran to curtail enrichment of uranium to limit any bomb-making potential. They want Tehran to cut its 19,000 uranium-enriching centrifuges to a few thousand.

Tehran, however, is insisting on vastly expanding capacity by adding thousands more centrifuges for what it says is strictly civilian energy purposes.

With talks scheduled to resume next week in Vienna, the dispute looms as the biggest threat to the comprehensive nuclear deal the two sides are trying to complete by a July 20 deadline.

Iran's demand to boost enrichment capacity "would be a showstopper," said Robert Einhorn, who was a member of President Barack Obama's inner circle of nuclear advisers until late last year and is now with the Brookings Institution. "There won't be an agreement."

The six world powers — France, Britain, Germany, the United States, Russia and China — are aiming for a deal that would curb Iran's nuclear development in exchange for easing the international sanctions that have crippled Iran's economy. Many nations fear that Iran, despite its denials, seeks to develop a bomb-making capability.

The goal of the world powers is to lengthen the time Iran would need to secretly enrich all the uranium necessary for a nuclear bomb, should it decide to build one in a crash program. U.S. officials estimate the so-called "breakout time" is now about two months. The West would like a year or more to ensure it could detect and stop the effort.

Iranian officials, including Supreme Leader Ayatollah Ali Khamenei, have repeatedly declared that they intend to expand, not dismantle, Iran's $100 billion nuclear infrastructure.

Obama remains cautious about the talks' prospects, saying the likelihood of success is 50-50 at best, White House officials say.

Despite these hurdles, a sizable group of diplomats and analysts believe chances for a deal are good. The key reason: Iran needs sanctions relief to bolster its weak economy, and the White House wants to ease a top-priority threat and score a major foreign policy victory.