The United States and five other world powers announced a landmark accord on Sunday that would temporarily freeze Iran's nuclear programme and lay the foundation for a more sweeping agreement.
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".