UNITED NATIONS (AP) — Seven months and more than a half dozen rounds into talks on a substantive nuclear deal, Iran and six world powers gathering for another session appear no closer to an agreement.

Iran claims its nuclear program has only peaceful purposes, but Western nations have long suspected Iran wants to have the capacity to make nuclear weapons.

The talks once again bring Iran to the negotiating table with the United States, Russia, China, Britain, France and Germany. But this time they are taking place on the sidelines of the U.N. General Assembly. That means U.S. Secretary of State John Kerry and his counterparts will likely join in, adding their diplomatic muscle to the meeting.

Ahead of the opening round Friday, chief U.S. negotiator Wendy Sherman acknowledged that the sides "remain far apart on other core issues, including the size and scope of Iran's uranium enrichment capacity." Depending on its level, enriched uranium can be used as reactor fuel or the fissile core of a nuclear warhead.

Iranian demands that it be allowed to keep its program at its present size and output are not acceptable and will not give Iran what it wants — an end to nuclear-related sanctions choking its economy, she told reporters.

"We must be confident that any effort by Tehran to break out of its obligations will be so visible and time-consuming that the attempt would have no chance of success," she said of Washington's push for deep, long-lasting cuts to prevent any quick move to a nuclear weapon-making mode.

Iranian Foreign Minister Mohamad Javad Zarif questioned sanctions, however, as effective in pressuring his country on its nuclear program, nothing it has greatly expanded over the past decades.

"The United States is obsessed with sanctions," he said.

Gary Samore of Harvard's Belfer Center, who was a part of the U.S. nuclear negotiating team until last year, suggested both sides may be waiting until closer to the deadline to make their moves. Ahead of that, he says, Iran appears to be exploiting other geopolitical tensions in hopes of gaining an advantage.

"The confrontation over Ukraine, the likes of (Islamic State group) — all that has led the Iranians to believe that they are in a stronger position than before," Samore said. "It doesn't seem to me like there has been any progress on the central question of enrichment."

Some facts about enrichment and other issues related to the nuclear negotiations:

URANIUM ENRICHMENT

There is agreement that Iran should have an "enrichment program with practical limits and transparency measures" to ensure it's peaceful. That has led to haggling over how many — and what kind — of centrifuges Iran should be allowed to have. The machines can enrich uranium from low, reactor-fuel level, all the way to grades used to build the core of a nuclear weapon, and their output grows according to how modern they are.

Iran has not publicly backed away from its plan to expand enrichment over the next eight years to a level that would require about 190,000 centrifuges. It now has about 20,000 centrifuges, half of them operational. Iranian officials have signaled they are ready to freeze that number for now. The United States wants Iran to have fewer than 1,000 centrifuges.

FORDO AND ARAK

The U.S. and its allies consider the underground enrichment plant near the Iranian village of Fordo a threat because it is heavily fortified against aerial attacks. They want it shut down or converted to non-enrichment functions. Among the Iranian offers rejected by the West is turning Fordo into an enrichment research facility.

The reactor under construction near the city of Arak is also a concern for the West because it is a heavy-water unit that would produce substantial amounts of plutonium that can be used as the fissile core of a missile. The Iranians have offered to re-engineer it to produce less plutonium — but that process is reversible. The U.S. seeks a completely new kind of reactor that produces only minuscule amounts of plutonium.

AGREEMENT LENGTH

An interim agreement says that if Iran honors a final agreement, it will eventually be treated as any other non-nuclear weapons member of the Nuclear Non-Proliferation Treaty. This means Iran would have the right to expand enrichment without having to worry about strict monitoring.

Senior U.S. officials define the number of years of restrictions as "in the double digits," while Iran wants it to be less than 10 years.

SUSPECTED NUCLEAR WEAPONS WORK

Iran denies wanting — or ever working on — nuclear weapons and has pledged to cooperate with the latest U.N. atomic agency effort to probe such allegations. But months into the inquiry, it has yet to provide information sought by the agency. While the investigation is separate from the talks, the U.S. says a deal can be struck only if the U.N. agency is satisfied with the probe and its final results.

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Associated Press diplomatic writer Matthew Lee contributed from Washington and AP U.N. chief correspondent Edith Lederer from the United Nations.