US President Barack Obama said that with the deal, “every pathway to a nuclear weapon is cut off” for Iran.
His Iranian counterpart, Hassan Rouhani, said it opened a “new chapter” in Iran’s relations with the world.
The deal reportedly gives UN nuclear inspectors extensive but not automatic access to sites within Iran.
Negotiations between Iran and six world powers – the US, UK, France, China and Russia plus Germany – began in 2006.
The so-called P5+1 want Iran to scale back its sensitive nuclear activities to ensure that it cannot build a nuclear weapon.
Iran, which wants crippling international sanctions lifted, has always insisted that its nuclear work is peaceful.
Mr Obama said that the deal “makes the world safer and more secure”, and that it provides for a rigorous verification regime. “This deal is not built on trust – it is built on verification,” he said.
EU foreign policy chief Federica Mogherini said the deal was “a sign of hope for the entire world”.
“It is a decision that can open the way to a new chapter in international relations,” she said, ahead of a final meeting between negotiators in Vienna.
Iranian Foreign Minister Mohammad Javad Zarif said the deal was “not perfect for anybody”, but that it was the “best achievement possible that could be reached”.
Iran’s President, Hassan Rouhani, tweeted: “With this unnecessary crisis resolved, new horizons emerge with a focus on shared challenges.”
The text of the deal is yet to be officially released but these are some of the details it is believed to contain:
- A compromise over the inspection of sites within Iran, the Associated Press quotes a diplomat as saying – UN inspectors would be allowed to monitor military sites but Iran could challenge requests for access
- Iran has accepted that sanctions could be restored in 65 days if it violates the deal, Reuters cited diplomats as saying
- A UN arms embargo and missile sanctions would remain in place for five and eight years respectively, Reuters reports
- Sanctions in areas including oil and gas trading, financial transactions, aviation and shipping will be lifted and billions of dollars of Iranian assets unfrozen, Iranian media report
‘Significant step forward’
Separately, the International Atomic Energy Agency (IAEA) and Iran said they had signed a roadmap to resolve outstanding issues.
IAEA head Yukiya Amano told reporters in Vienna that his organisation had signed a roadmap “for the clarification of past and present outstanding issues regarding Iran’s nuclear programme”.
He called the agreement a “significant step forward”, saying it would allow the agency to “make an assessment of issues relating to possible military dimensions to Iran’s nuclear programme by the end of 2015”.
There has been stiff resistance to a deal from conservatives both in Iran and the US.
Israel’s government has also warned against an agreement.
Prime Minister Benjamin Netanyahu said it was a “historic mistake” that would provide Iran with “hundreds of billions of dollars with which it can fuel its terror machine and its expansion and aggression throughout the Middle East and across the globe”.
Iran’s enemies remain of the view that the Iranians are hell-bent on acquiring nuclear weapons at some point and have merely agreed to a delay in return for a variety of short-term concessions.
There is a danger now that Saudi Arabia will feel that a nuclear-capable Shia state must be matched by a nuclear capability in the hands of the Sunni states too.
That brings the nightmare of a nuclear arms race in the Middle East a step closer.