The 55-year old career diplomat led Iran's negotiating team in concluding a deal with world powers in Vienna on Tuesday after almost two years of negotiations, to curb Iran's nuclear programme in exchange for sanctions relief after decades of tensions with the West.

But Zarif, who has spent long spells studying or working in the United States, needed the support of President Hassan Rouhani, and the cautious backing of supreme leader Ali Khamenei, to protect him from domestic opponents whose distrust of the West has characterised Iranian policy since the 1979 Islamic Revolution.

"Zarif is the most effective diplomat Iran has had since the revolution," said Karim Sadjadpour, an analyst at the Carnegie Endowment for International Peace. "He is the only man in the world that can have a conversation with (US Secretary of State) John Kerry one day and Ali Khamenei the next, and reassure both of them that he shares their end game," he added.

Zarif's easy smile and mastery of English helped him build up a rapport with foreign diplomats, particularly Kerry, with whom he is on first-name terms. He also shunned the bombastic and confrontational language that had become the hallmark of the Islamic Republic's officials.

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