Iranians celebrated in the streets after negotiators reached a framework for a nuclear accord and U.S. President Barack Obama hailed an "historic understanding", but senior global diplomats cautioned that hard work lies ahead to strike a final deal.
The tentative agreement, struck on Thursday after eight days of talks in Switzerland clears the way for a settlement to allay Western fears that Iran could build an atomic bomb, with economic sanctions on Tehran being lifted in return.
It marks the most significant step toward rapprochement between Washington and Tehran since the 1979 Iranian revolution and could bring an end to decades of Iran's international isolation.
But the deal still requires experts to work out difficult details before a self-imposed June deadline and diplomats said it could collapse at any time before then.
"We are not completely at the end of the road and the end of the road should be in June,” said French Foreign Minister Laurent Fabius "Nothing is signed until everything is signed, but things are going in the right direction."
Israeli Prime Minister Benjamin Netanyahu who has the ear of U.S. Republicans who control both houses of Congress, said the powers negotiating with Iran must add a new demand that Tehran recognize Israel's right to exist.
Under Thursday's terms, Iran would cut back its stockpiles of enriched uranium that could be used to make a bomb and dismantle most of the centrifuges it could use to make more. Intensive international inspections would prevent it from violating the terms in secret. Washington said the settlement would extend the "breakout time" needed for Iran to make a bomb to a full year, from 2-3 months now.
For Iran, it would eventually lead to the end of sanctions that have cut the oil exports that underpin its economy by more than half over the past three years.
Still, decades of hostility remain between countries that have referred to each other as "the Great Satan" and part of the "axis of evil". Obama and Iranian President Hasan Rouhani, who both took risks to open the dialogue, will each have to sell the deal to skeptical conservatives at home.
U.S. Republicans have demanded that the Congress they control be given the right to review the deal.