Denounced by Israel as a "bad deal", a breakthrough agreement between Iran and six world powers to restrain its nuclear program should nevertheless make it significantly harder for the Islamic state to build any atomic bomb.

By halting Iran's most sensitive enrichment of uranium, Sunday's interim accord is designed to stop the expansion of Iranian atomic activities and buy time for negotiations on a final settlement of the decade-old nuclear dispute.

However, Iran will for now retain thousands of centrifuges refining uranium - albeit only to concentrations far below that needed for nuclear weapons - and a stockpile that could potentially be used for bombs if processed much more.

"The short-term deal accomplishes a great deal," nuclear proliferation expert David Albright of the US Institute for Science and International Security (ISIS) said.

For example, he said, it would eliminate Iran's stock of uranium gas refined to a fissile purity of 20 percent, a source of deep concern for the West as it represents a relatively short technical step away from bomb-grade material.

Under the agreement, Iran must halt this higher-grade enrichment and also dilute or convert its existing reserve of such uranium to a form that is not suitable for further enrichment, according to a US fact sheet.

Once this is done, the breakout time - how long it would take Iran to produce sufficient highly-enriched uranium for one atomic bomb - would lengthen from at least 1-1.6 months to at least 1.9-2.2 months if the Iranians used all their installed centrifuges, Albright said in an e-mail.

"This may seem a small increase, but with the IAEA daily checking the camera film at Natanz and Fordow, this increase in breakout times would be significant," he said, referring to the UN nuclear watchdog, the International Atomic Energy Agency.

Iran has committed to grant IAEA inspectors daily access to its underground enrichment facilities at Natanz and Fordow, its most controversial nuclear sites, the US fact sheet said.

"This access will provide even greater transparency into enrichment at these sites and shorten detection time for any non-compliance," it said.

The UN watchdog, tasked with ensuring that no nuclear material is diverted for military purposes in member states, is currently believed to visit these plants about once a week.

"This agreement virtually eliminates the possibility of Iran dashing towards a nuclear weapon without prompt detection by the UN nuclear inspectors," said Ali Vaez of the International Crisis Group think-tank.

But analysts caution that no one can rule out the existence of secret nuclear sites in Iran without it agreeing to let the IAEA conduct snap inspections anywhere beyond declared atomic installations under the agency's Additional Protocol regime.

Diplomats say they have no clear indication that Iran harbors any such clandestine facility now but - given Tehran's previous concealment of some nuclear sites from the IAEA - world powers are expected to seek Iranian adoption of the Additional Protocol as part of a broad, final settlement.

Refined uranium can be used to fuel nuclear power plants - Iran's stated goal - but can also provide the fissile core of an atomic bomb. Iran denies accusations that it is seeking the capability to make nuclear weapons.

"The first phase of the agreement ensures that Iran will not make any further progress towards a bomb while we continue to negotiate a complete end to the program," said Joseph Cirincione of the Ploughshares Fund group, which opposes the spread of nuclear, biological and chemical weapons.

Israel, believed to be the Middle East's only nuclear-armed state, made clear already before this week's talks in Geneva between Iran and the United States, Russia, France, Britain, Germany and China that it opposed the proposals on the table.

The Jewish state sees Iran as a mortal threat and wants it to dismantle all of its enrichment infrastructure.

The agreement "grants Iran exactly what it wanted - both a significant easing in sanctions and preservation of the most significant parts of its nuclear program," an official in Prime Minister Benjamin Netanyahu's office said.

But proliferation expert Mark Fitzpatrick of the International Institute for Strategic Studies, a think-tank in London, said the deal "significantly sets back" Iran's ability to produce a nuclear weapon without being detected and stopped.

"Just a few weeks ago analysts were projecting that by next summer Iran might be able to produce a weapon's worth of uranium within a week or so," Fitzpatrick said.

"Now it will be months. Yes, enrichment will continue - that was inevitable - but it will be more tightly inspected."

Still, verification will be "full of landmines", warned Mark Hibbs of the Carnegie Endowment think-tank.

"This will require a level of cooperation and information sharing between the IAEA, the powers and Iran which is probably unprecedented concerning one country's nuclear program," Hibbs said.

The agreement text has not been made public. But the US fact sheet said Iran had also agreed not to hook up additional centrifuges of any type, leave inoperable roughly half of installed such machines at Natanz, and not to increase its stockpile of 3.5 percent enriched uranium, among other things.

According to the IAEA's latest report on Iran, the country already has more than 7,000 kg (15,400 pounds) of low-enriched uranium, an amount experts say could be enough for four bombs if refined to 90 percent fissile concentration.

After sharply expanding its nuclear program since it started refining uranium in 2007 in defiance of UN Security Council resolutions, Iran now has about 19,000 installed centrifuges, of which about half are operating. That would not appear to change much under Sunday's interim agreement.

The United States said Iran had also agreed to no further advances in construction of its Arak heavy-water reactor, which could produce plutonium once operational - an alternative fissile source for atomic bombs.

Albright said "very tough" issues remained to be negotiated to achieve a long-term comprehensive agreement that would ensure Iran that does not build nuclear weapons.
"Iran and the United States remain far apart. What will be the exact limits on the size and scope of Iran's centrifuge program?" he asked.

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