Iranian Foreign Minister Mohammad Javad Zarif, left, reacts next to EU foreign policy chief Catherine Ashton as Secretary of State John Kerry embraces French Foreign Minister Laurent Fabius after a statement on the deal. (FABRICE COFFRINI/AFP/Getty Images)

The plan accepted by Iran on Sunday would accomplish something that U.S. governments have sought in vain for more than a decade: A pause, at least, in Iran’s inexorable march to a nuclear-weapons capability.

The historic agreement, described as a first step toward a more comprehensive nuclear deal six months from now, freezes or reverses progress in nearly every aspect of Iran’s nuclear program, from the installation of new centrifuges to work on critical components of a heavy-water reactor that could someday provide Iran with a source of plutonium.

And, while Iran would be permitted to enrich uranium at low levels, its total stockpile of nuclear fuel would be frozen at current amounts, administraiton officials said. That freeze, combined with dramatically increased monitoring of Iranian facilities, would make it virtually impossible for Iran to work on a nuclear weapon without being detected, the officials said. In return, Iran will receive modest sanctions relief and access to some of its frozen accounts overseas.

“The purpose of this is simple: To require Iran to prove the peaceful nature of its nuclear program and ensure that it cannot acquire a nuclear weapon,” Secretary of State John F. Kerry said.

Kerry said the first six-month phase of the deal would improve the security of U.S. allies in the region while efforts were made to negotiate a more comprehensive agreement. “This first step actually rolls back the program from where it is today and enlarges the breakout time,” he said.

President Obama says the U.S. has agreed to provide Iran with "modest relief" from sanctions as part of a deal on the country's nuclear program. (Associated Press)

The agreement reached on Sunday in Geneva after four days of marathon talks has drawn harsh reviews from Israel as well as congressional skeptics who believe Western diplomats should have pushed for far deeper concessions from Iran, including a halt to all uranium enrichment and a dismantling of Iran’s nuclear facilities. The Obama administration says deeper cuts will occur in the second phase of the agreement, after a critical confidence-building phase that allows the sides to build trust.

Key details of the proposal, which had been kept confidential while the negotiations were underway, were publicly unveiled as diplomats concluded four days of intense bargaining over the final shape of the deal.

The concessions accepted by Iran include numerous curbs on the country’s uranium enrichment program, the source of most Western concerns about Tehran’s nuclear ambitions since the discovery of a partially completed enrichment plant near the city of Natanz.

Under the terms of the deal, Iran would stop installing new centrifuges, and also refrain from using the thousands of centrifuges that have been installed but are not yet enriching uranium —meaning Iran could use only about half of the roughly 18,000 centrifuges it currently possesses.

Those centrifuges would be limited to making only low-enriched uranium, of the kind used in nuclear power plants. While Iran would continue to make the nuclear fuel, its total stockpile in six months would not be allowed to grow beyond current levels. In practice, Iran would face a choice of either halting enrichment or converting its uranium into metal fuel plates.

In a key concession, Iran agreed to halt all production of so-called 20-percent-enriched uranium, a type of fuel that can be easily converted to highly enriched uranium used in nuclear bombs. Iran’s entire stockpile of 20-percent fuel—just under 450 pounds—would have to be neutralized through conversion into metal or blending with natural uranium to reduce its purity.

A sticking point during the talks involved Iran’s continued work on a partly constructed heavy water reactor near the town of Arak. If allowed to operate, the reactor could supply Iran with a potential source of plutonium, which, like high-enriched uranium, can be used to make nuclear bombs. Under the agreement reached in Geneva, Iran would be required to halt work on building fuel rods or other components for the facility.

To ensure compliance with the agreements, Iran’s nuclear facilities would be subject to unprecedented monitoring, with daily visits by international inspectors who also would have access to recordings by remote video equipment.

In return, Iran would receive economic incentives that would be initially modest, with the prospects of more substantial sanctions relief under the comprehensive deal to be negotiated by next spring. Western diplomats estimated the value of the relief package at about $7 billion over the six months that the interim agreement is in place.

Iran would be given access to about $4.2 billion dollars of its foreign currency holdings, now frozen in banks overseas. Western governments also would ease restrictions affecting Iran’s trade in petrochemical products, precious metals, and airplane and automobile parts.

Western diplomats said the revenue gained by Iran under the deal would not begin to offset losses to the economy from Western sanctions on Iran’s oil and banking sectors—restrictions that would remain in place until Iran agrees to the comprehensive deal on more sweeping restrictions to its nuclear program.

The short-term sanctions would be reversed if Iran fails to comply with the agreement, diplomats said.