After months of negotiations, Iran and the major world powers have agreed on the framework of a deal to address Western concerns about Tehran's nuclear programme.
The framework agreement reached in Geneva on Thursday is aimed at curbing Iran's nuclear programme for at least a decade, and is a step towards a final pact that could end 12 years of brinkmanship, threats and confrontation.
The tentative agreement, after marathon talks in Switzerland, clears the way for negotiations on a settlement aimed at allaying Western fears that Iran is seeking to build an atomic bomb.
The framework is contingent on reaching a final agreement by 30 June. All sanctions on Iran will remain in place until a final deal.
Celebrations erupted in the Iranian capital Tehran today - videos and pictures posted on social media showed cars in Tehran honking horns as passengers clapped.
President Barack Obama described the agreement as a ''historic understanding with Iran'' and compared it to nuclear arms control deals struck by his predecessors with the Soviet Union that ''made our world safer'' during the Cold War. He also cautioned, however, that ''success is not guaranteed.''
Many details still need to be worked out. Diplomats close to the negotiations said the deal was fragile.
Under the outline deal, Iran would shut more than two-thirds of its installed centrifuges capable of producing uranium that could be used to build a bomb, dismantle a reactor that could produce plutonium, and accept intrusive verification.
The negotiations between Iran and the so-called P-5+1 powers - Britain, China, France, Germany, Russia and the United States – have created a framework of settlement that is strongly opposed by Israel (See: Netanyahu blasts Iran nuclear deal, convenes cabinet)
Iran agreed to significantly reduce the number of installed uranium enrichment centrifuges it has to 6,104 from 19,000 and will only operate 5,060 for 10 years under the future agreement with the six powers. According to a US fact sheet. Iran will only use first-generation centrifuges during that time, it said.
One of the most sensitive issues during the negotiations, Iran's research and development work, will also be limited.
''Iran has agreed to not conduct research and development associated with uranium enrichment at Fordow for 15 years,'' the US fact sheet said. It also noted that Iran will remove the 1,000 more advanced second-generation centrifuges currently installed at Natanz and place them in International Atomic Energy Agency-monitored storage for 10 years.
Under Thursday's agreement, Iran will gradually receive relief from US and European Union nuclear sanctions if it complies with the terms of a final deal. Some UN Security Council sanctions would be gradually lifted, though others would remain in place, specifically those relating to proliferation.
''We're still some time away from reaching where we want to be,'' said Iran's foreign minister Mohammad Javad Zarif.
Failure to comply with terms of the deal will cause the US and EU sanctions to ''snap back into place'', the US fact sheet said. It was less specific on UN sanctions, one of the main sticking points in the negotiations, saying only that they could be re-imposed in the event of Iranian non-compliance.
US secretary of state John Kerry told reporters many technical details needed to be worked out, including the possible lifting of a UN arms embargo and the modernization of the Arak heavy-water reactor and Fordow underground sites.