ISTANBUL -- The U.S. is joining five other world powers for talks with Iran this week publicly confident that international efforts have slowed Tehran's capacity to make nuclear arms and created more time to press Tehran to accept curbs on its atomic activities.

But while diplomats and officials at the International Atomic Energy Agency -- the U.N. nuclear monitor -- agree that Iran's enrichment program has struggled over past years, the Federation of American Scientists warns against complacency.

It notes impressive improvements in the performance of the Iranian machines that enrich uranium -- an activity that has provoked U.N. sanctions because it could be used to make nuclear weapons.

Washington's message is essentially this: Iran is struggling with uranium enrichment, a process that can create both nuclear fuel and fissile warhead material. Significantly, that view is backed by Israel, Iran's implacable foe and considered to have the Mideast's best intelligence on Iran's nuclear strivings.

If true, that leaves more time to negotiate in hopes Iran will come around and give up enrichment -- thereby removing the threat of an Israeli or U.S. military strike on Iran's nuclear facilities.

But in a study shared ahead of publication with The Associated Press the Washington-based FAS argues that Iran last year appears to have increased efficiency of the machines that produce enriched uranium by 60 percent, giving it the technical capacity to produce enough material for a simple nuclear warhead in 5 months.

Iran insists it is enriching only to make nuclear fuel and study author Ivanka Barzashka emphasizes Tehran is unlikely to provoke the world -- and increase the likelihood of attack -- by kicking out IAEA inspectors and re-calibrate their centrifuges from making low-enriched to weapons grade uranium.

Olli Heinonen, who retired late last year as the IAEA deputy director general in charge of the agency's Iran file, described the likelihood of such a "breakout scenario" as a "suicidal mission" and noted that manufacturing nuclear warhead material is only one step in making a weapon.

At the same time, he said he cannot "dispute the correctness of the figures" in the study.

With the two sides coming to the table at Istanbul as far apart as they were at the end of their first round in Geneva last month, Barzashka says that efforts to bridge the divide between the two sides must be increased.

"The biggest issue with recent statements that Iran's nuclear drive has been slowed down is that we are getting a false sense of security that we have bought more time," Barzashka said in an e-mail. "That takes away from the urgency ... (of) a diplomatic breakthrough."

Barzashka based her conclusions on data of nuclear material fed into enriching centrifuges and the output of these machines collected by the International Atomic Energy Agency -- the U.N. monitor of Iran's nuclear program.

An IAEA official who read Barzashka's 14-page study described her conclusion of impressive progress in output of enriched uranium as "very solid," and "based on the best possible data." But that official and a senior diplomat based in Vienna and familiar with Iran's enrichment strivings said that -- overall -- the centrifuges continued to perform substantially below capacity.

The two asked for anonymity because they were not authorized to comment to the media.

The FAS study says the increased output by thousands of centrifuges at Iran's centrally located underground facility at Natanz could be either due to better recovery of previously wasted feedstock or increased efficiency of the centrifuges that spin gas into enriched uranium.

"Contrary to statements by U.S. officials and many experts, Iran does not appear to be slowing down its nuclear drive," it says.

Such views contrast with the public optimism expressed by Washington ahead of the Istanbul talks convened by the EU and grouping Iran on one side of the table and the U.S. Russia, China, Britain, France and Germany on the other.

Israeli officials now talk of a three-year window -- 2014 -- before Iran can make a bomb. That compares with projections of 2011 just three years ago, and U.S. Secretary of State Hillary Clinton told NBC's Today show on Wednesday that the new Israeli estimates are "very significant."

The delay, she said "gives us more of a breathing space to try to work to prevent them from obtaining a nuclear weapon."

Two outside forces would account for any Iranian problems in enriching uranium -- the increasing weight of U.N. sanctions, meant to choke off raw materials needed to make and maintain the program; and the apparent havoc caused by the mysterious Stuxnet computer malware.

Iran has acknowledged that Stuxnet hit "a limited number of centrifuges", saying its scientists discovered and neutralized the malware before it could cause any serious damage. The worm is assumed to have caused disruption of enrichment in November that temporarily crippled thousands of centrifuges at Natanz.

Barzashka said that -- while the sanctions might have slowed Iran's ability to develop, new, and more efficient centrifuges -- they do not seem to have slowed improvements in the output of the present generation of machines used at Natanz.

Ahead of the talks, Iran is trying to take the diplomatic offensive. It is pushing an agenda that covers just about everything except its nuclear program: global disarmament, Israel's suspected nuclear arsenal and Tehran's concerns about U.S. military bases in Iraq and elsewhere in the region.

"Let them issue 100,000 resolutions," Iranian President Mahmoud Ahmadinejad said Tuesday referring to U.N. Security Council sanctions and other efforts to curb Iran's nuclear program. "It's not important. Let them say what they want to."

Such statements appear to leave scant maneuvering room for the six nations and their ultimate goal: to get Iran to halt uranium enrichment.

The U.S. and others fear Iran's enrichment program could eventually to lead nuclear weapons. Iranian officials say they only want reactors for energy and research -- and that the Nuclear Nonproliferation Treaty gives Iran the legal right to enrich uranium and produce nuclear fuel.

On Monday, chief Iranian nuclear negotiator Saeed Jalili said "common points" shared by both sides have to be discussed in Istanbul if any progress is expected, not unilateral demands from the U.S. and its allies.

Iran's U.N. ambassador, Mohammad Khazaee, repeated that Iran will "never negotiate on our inalienable right to use nuclear energy for ... peaceful purposes."

"It doesn't mean that Iranians are looking for confrontation," he told reporters in New York Tuesday. "But at the same time ... it's not going to work to put a knife in the neck of somebody, or your sword, and at the same time asking him to negotiate with you."

Uranium enrichment lies at the heart of the dispute. Low-enriched uranium -- at around 3.5 percent -- can be used to fuel a reactor to generate electricity, which Iran says is the intention of its program. But if uranium is further enriched to around 90 percent purity, it can be used to develop a nuclear warhead.

Iran's ambassador to the U.N.'s International Atomic Energy Agency, Ali Asghar Soltanieh, says the Istanbul talks are a "window for an honorable path for the West to get out of the present impasse."

But Christopher Hill, a former US assistant secretary of state for east Asia, says sanctions should be a tool of diplomacy.

"Just as the US adopted a 'bomb and talk' approach with the Serbs during the denouement of the Bosnian war, America must be willing to 'sanction and talk' when it comes to Iran, thereby creating greater space for an eventual diplomatic strategy," he said.