It was the first time in nearly a decade, American officials said, that an international agreement had been reached to halt much of Iran's nuclear programme and roll some elements of it back.
The aim of the accord, which is to last six months, is to give international negotiators time to pursue a more comprehensive pact that would ratchet back much of Iran's nuclear programme and ensure that it could be used only for peaceful purposes.
Iran agreed to stop enriching uranium beyond 5 per cent, a level that would be sufficient for energy production but that would require further enrichment for bomb-making. To make good on that pledge, Iran will dismantle links between networks of centrifuges.
Its stockpile of uranium enriched to 20 per cent, a short hop from weapons-grade fuel, would be diluted or converted into oxide so that it could not be readily used for military purposes. Iran agreed that it would not install any new centrifuges, start up any that are not already operating or build new enrichment facilities.
The agreement, however, does not require Iran to stop enriching uranium to a low level of 3.5 per cent, or to dismantle any of its existing centrifuges.
Shortly after the agreement was signed in Geneva, President Barack Obama hailed it as the most "significant and tangible" progress of a diplomatic campaign that began when he took office.
"Today, that diplomacy opened up a new path toward a world that is more secure," he said, "a future in which we can verify that Iran's nuclear programme is peaceful and that it cannot build a nuclear weapon."
In Geneva, Iranian Foreign Minister Mohammad Javad Zarif said he hoped the agreement would lead to a "restoration" of trust between Iran and the United States.
Secretary of State John Kerry, who flew to Geneva Saturday, said the deal would "require Iran to prove the peaceful nature of its nuclear programme".
The accord was a disappointment for Israel, which had urged the United States to pursue a stronger agreement that would lead to a complete end to Iran's enrichment programme. But Iran made it clear that continuing enrichment was a prerequisite for any agreement.
The United States did not accept Iran's claim that it had a "right to enrich" under the nuclear non-proliferation treaty. But American officials signalled last week that they were open to a compromise in which the two sides would essentially agree to disagree on how the proliferation treaty should be interpreted, while Tehran continued to enrich.
In return for the initial agreement, the United States agreed to provide $6 billion to $7 billion in sanctions relief. Of this, roughly $4.2 billion would be oil revenue that has been frozen in foreign banks.
This limited sanctions relief can be accomplished by executive order, allowing the Obama administration to make the deal without having to appeal to Congress, where there is strong criticism of any agreement that does not fully dismantle Iran's nuclear programme. The fact that the accord would only pause the Iranian programme was seized on by critics who said it would reward Iran for institutionalising the status quo.
Obama addressed those concerns in his speech, insisting that the easing of sanctions could be reversed if Iran failed to reach a final agreement or reneged on the terms of this one. "Nothing will be agreed to unless everything is agreed to," he said.
He also noted the qualms of Israel, Saudi Arabia and other Persian Gulf allies of the United States, saying they "had good reason to be sceptical of Iran's intentions". But he said he had a "profound responsibility" to test the possibilities of a diplomatic solution.
In Geneva, Kerry said of the agreement: "It will make our partners in the region safer. It will make our ally Israel safer."
The deal would also add at least several weeks, and perhaps more than a month, to the time Iran would need to produce weapons-grade uranium, according to estimates by nuclear experts. American officials argued that it would preclude Iran from shortening the time it would need to produce enough bomb-grade uranium for a nuclear device even further, and would provide additional warning if Iran sought to "break out" of its commitment to pursue only a peaceful nuclear programme.
A second and even more contentious debate centred on whether an initial deal would, as the Obama administration said, serve as a "first step" toward a comprehensive solution of the nuclear issue, one that would leave Iran with a peaceful nuclear programme that could not easily be used for military purposes.
Some experts, including a former official who has worked on the Iranian issue for the White House, said it was unlikely that Iran's supreme leader, Ayatollah Ali Khamenei, would ever close the door on the option to develop nuclear weapons. Instead, they said, any initial six-month agreement is more likely to be followed by a series of partial agreements that constrain Iran's nuclear activities but do not definitively solve the nuclear issues.
The agreement also reflected compromises on other issues. On the contentious issue of the heavy water reactor Iran is building near Arak, Iran agreed not to produce fuel for the plant, install additional reactor components there or put the plant into operation. Iran is not required to dismantle the facility, however, or convert the plant into a light water reactor that would be less useful for military purposes.
Regarding enrichment, Iran's stockpile of such low-enriched uranium would be allowed to temporarily increase to about eight tonnes from about seven tonnes currently. But Tehran would be required to shrink this stockpile by the end of the six-month agreement back to seven tonnes. This would be done by installing equipment to covert some of that stockpile to oxide.
To guard against cheating, international monitors would be allowed to visit the Natanz enrichment facility and the underground nuclear enrichment plant at Fordo on a daily basis. But Iran did not agree to all of the intrusive inspection regime that the International Atomic Energy Agency had said was needed.
The key points
* Iran to halt uranium enrichment above 5%... But allowed to 'neutralise' its stockpile of 20% enriched uranium and keep it.
* Iran to build no new centrifuges... But allowed to keep its two main enrichment facilities in operation.
* Iran to suspend work at the heavy-water Arak reactor... But not required to dismantle the facility.
* Iran to allow monitors to visit Natanz, Fordo facilities...
But IAEA's call for an intrusive inspection regime not accepted.