Iran's arch-foe Israel quickly lashed out at the agreement, which Prime Minister Benjamin Netanyahu described as "a historic mistake for the world".
The breakthrough came on the 18th day of marathon talks in Vienna between Tehran and the so-called P5+1 -- the United States, Russia, China, Britain, France and Germany.
"The agreement is concluded," a diplomat close to the discussions told AFP in the Austrian capital, where a final ministerial meeting between Iran and the world powers was under way to adopt the deal.
The head of the UN atomic watchdog said he had signed a "roadmap" with Iran for probing suspected efforts to develop nuclear weapons, a key part of an overall accord.
International Atomic Energy Agency (IAEA) chief Yukiya Amano said he aimed to issue a report on the watchdog's investigation by December 15.
The landmark deal is expected to sharply curb Iran's nuclear programme and impose strict UN inspections in order to make any drive to make nuclear weapons all but impossible and easily detectable.
In return, the web of UN and Western sanctions choking Iranian oil exports and the economy of the country of 78 million people would be progressively lifted.
Iran has always rejected the allegations as based on faulty intelligence provided by its enemies to a gullible and biased IAEA, and a probe has been stalled since last year.
The diplomatic push began when Iranian President Hassan Rouhani came to power in 2013. In November that year an interim deal was agreed but two deadlines in 2014 for a lasting accord were missed.
Then in April, the parties scored a major breakthrough by agreeing the main outlines of an accord, aiming to finalise it by June 30, a deadline since pushed back twice.
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