Leading American scientists Friday warned against Western complacency over Iran's nuclear drive, saying in a study that Tehran last year boosted its capacity to build an atomic bomb.

The study, published by a body set by scientists from the top-secret Manhattan Project which built the world's first atomic bomb, comes as the United States and five other powers hold nuclear talks with Iran in Istanbul.

And it follows claims by US and Israeli officials that international efforts have slowed Iran's nuclear drive.

The Washington-based Federation of American Scientists said on its website that the gas centrifuges at Iran's main enrichment plant in Natanz became more efficient in 2010.

Centrifuges are the machines that enrich uranium for fuel in either civilian nuclear power plants or for a bomb's destructive power. Uranium must be enriched to 90 percent for a bomb, compared to five percent for power plants.

"Despite a drop in centrifuge numbers during 2010, the total enrichment capacity of Iran's main facility has increased relative to previous years," said the study authored by Ivanka Barzashka.

"The growth in enrichment capacity from 2009 to 2010 is greater than from 2008 to 2009," it said, adding the calculations were made on data provided by the UN nuclear watchdog, the International Atomic Energy Agency.

"Contrary to statements by US officials and many experts, Iran clearly does not appear to be slowing down its nuclear drive. On the contrary, it has a greater enrichment capacity and seems to be more efficient at enrichment," it said.

Barzashka estimated "it would take Iran anywhere from five months to almost a year to produce enough HEU (highly enriched uranium) for a single crude bomb, which does not seem like a viable breakout option."

"Breakout" potential is the time it would take to build a bomb.

"We are still in a stage where the numbers of new centrifuges Iran installs and their effective performance have significant effect on its time to a bomb," the author said.

During a January 10 visit to Abu Dhabi, on the opposite side of the Gulf from Iran, Clinton said international sanctions have made it "much more difficult" for Iran to pursue its nuclear ambitions.

And Israel's strategic affairs minister, Moshe Yalon, said last month that a series of "technological challenges and difficulties" meant Tehran was still about three years away from being able to build nuclear weapons.

The New York Times reported January 16 that US and Israeli intelligence services collaborated to develop a destructive computer worm, known as Stuxnet, to sabotage Iran's atomic bomb-making efforts.

Iran insists its nuclear program is for peaceful purposes, denying Western charges it seeks to build bombs.

earlier related report
Iran in new nuclear wrangling with world powers
Istanbul (AFP) Jan 21, 2011 - Six world powers began fresh talks with Iran Friday in a bid to resolve tensions over its controversial nuclear drive, but the United States warned against building up hopes of a breakthrough.

EU foreign policy chief Catherine Ashton and Iran's top nuclear negotiator, Saeed Jalili, led the delegations which convened for the two-day meeting at a seafront Ottoman palace in Istanbul, a Turkish diplomat said.

Ashton represented the so-called P5+1 group of Britain, China, France, Russia, the United States and Germany.

The United States, which suspects Iran's nuclear programme masks efforts to develop an atomic bomb, tamped down expectations for the talks but stressed the need for Iran to engage in a "credible" process to dispel the suspicions.

"We're not expecting any big breakthroughs," Mark Toner, a State Department spokesman, said in Washington ahead of the meeting.

"But we want to see a constructive process emerge that... leads to Iran engaging with the international community in a credible process and engaging and addressing the international community's concerns about its nuclear program," he said.

The Istanbul meeting was the second round after negotiations between the P5+1 group and Iran resumed last month in Geneva after a 14-month hiatus.

Bruno Tertrais, a French analyst specialising on Iran, described the meeting as "not negotiations but an attempt to find a way to resume the negotiations."

"The Iranians are playing for time -- that is their main strategy. The P5+1 has no illusion," he said.

In the eve of the talks, Russia -- which for the past decade has been building Iran's sole nuclear power plant -- called for discussions on lifting UN sanctions on Tehran, after US Secretary of State Hillary Clinton suggested Washington might consider fresh unilateral sanctions.

"The nuclear programme must be at the heart of the discussions... but there's not only one topic for this meeting, the lifting of sanctions on Iran must also be on the agenda," Russian Foreign Minister Sergei Lavrov said.

Russia and China, one of Iran's big trading partners, had backed all four sets of UN sanctions against Tehran.

The talks are aimed at ascertaining whether Iran is seeking nuclear weapons or is indeed looking only to meet the energy needs of its growing population, as it insists.

Iran is under four sets of UN sanctions over its refusal to suspend uranium enrichment, the sensitive process which can be used to make nuclear fuel or, in highly extended form, the fissile core of an atom bomb.

The US and the EU have imposed a series of their own unulateral sanctions on Tehran.

Iranian President Mahmoud Ahmadinejad has called for the lifting of the sanctions if the West wanted to see progress in the talks.

He raised the bar on Wednesday, telling a cheering crowd at home that Tehran would not back down from its nuclear programme.

"They say: 'We want negotiation'... You are free to choose the path (of either cooperation or confrontation), but bear in mind that by adopting the old path (of confrontation), you will face a more scandalous defeat," he said.

"You could not stop us from being nuclear ... The Iranian nation will not retreat an inch. The nuclear issue is over from the Iranian point of view."

Iran's atomic chief Ali Akbar Salehi has gone further, insisting Tehran will not even discuss its "nuclear dossier" at the Istanbul meeting, a tactic that Tehran has employed ahead of earlier talks with the powers.