"If Iran cheats, the world will know it," Obama said, adding that the deal was based not on trust but on "unprecedented verification", according to a media report.

He said the framework agreement had come after "months of tough, principled diplomacy", and that it was "a good deal".

Under its terms, Iran would reduce its stored centrifuges by two thirds, and be prevented from developing weapons-grade plutonium, Obama said. In return, there would be a "phased" listing of sanctions against Iran.

European Union (EU) Foreign Policy Chief Federica Mogherini, speaking at a news conference alongside Iranian Foreign Minister Mohammad Javad Zarif after agreement was reached, said a "decisive step" had been achieved.

"We have reached solutions on key parameters of a joint comprehensive plan of action," she said.

Negotiators would now start "drafting the text" of the plan "guided by the solutions", Mogherini added.

Earlier, in a tweet, the Iranian Foreign Minister said: "Found solutions, ready to start drafting immediately".

Also on Twitter, Germany's Foreign Ministry said: "Agreement for framework on final agreement reached".

US Secretary of State John Kerry tweeted: "Big day... Back to work soon on final deal".

A comprehensive deal is expected by the end of June.

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