Even before he became secretary of state, John Kerry vowed to stop Iran from gaining an atomic bomb and warned the clock was ticking on reining in its nuclear ambitions.

But few if any of his fellow senators were aware during his January 2013 confirmation hearing that Kerry had already made tentative steps to try to bring Iran back to nuclear talks as part of a secret overture by President Barack Obama.

Just over two years later, Kerry and his team of diplomats and experts have pulled off a tough assignment -- laying out the contours of a deal to sharply cut back Tehran's nuclear program.

Even though the final accord has yet to be agreed, it is a huge achievement for the 71-year-old who took over as America's top diplomat in February 2013, a month after calling for "fresh thinking" to resolve global problems at his confirmation hearing.

The son of a diplomat, who grew up in the rubble of post-World War II Europe, Kerry has diplomacy running through his DNA.

After his failed 2004 bid for the presidency, serving as secretary of state is the pinnacle of his career.

On taking office, Kerry swiftly signalled that he was a different kind of diplomat from his predecessor Hillary Clinton, who had won rock star treatment during her record-breaking travels.

The lanky former lawyer, who served for 29 years as a senator for Massachusetts, was keen to roll up his sleeves and delve deep into some of the world's most intractable problems.

He took over just as the first flush of the Arab Spring began to fizzle, replaced by upheavals and chaos in countries like Egypt, Libya and Syria and as key allies like Jordan and Turkey struggled under the strain.

- Bulging in-tray -

The shifting geopolitical landscape also saw the surprise growth of the Islamic State (IS) group, an ultra-violent jihadist faction which almost out of nowhere captured swathes of Iraq and Syria last year and declared a caliphate.

In two short years Kerry has found himself juggling several huge global crises, while having already personally invested himself in the holy grail of diplomacy -- a Middle East peace deal.

While his quest to reconcile Israel and the Palestinians failed spectacularly last year, Kerry is credited with having helped to negotiate with Russia a deal to rid Syria of chemical weapons.

He has also had to deal with building a coalition to fight the IS group, ease fears that an election standoff in Afghanistan could spark new turmoil and seek an end to the conflict in eastern Ukraine.

Then Obama handed him just one more dossier.

The 2013 election of new Iranian President Hassan Rouhani provided a fresh opportunity to seek once again to put a nuclear bomb out of Iran's reach.

As a senator Kerry had been part of early secret 2011-2012 meetings with the former Iranian government in Oman, so he was the natural choice to explore whether a nuclear deal with Iran may now be possible after a decade-long standoff.

Within weeks, he was criss-crossing the globe for days of intense meetings with his new Iranian counterpart Mohammad Javad Zarif, gaining an invaluable insight into the new leadership of a country which remains a US foe.

- Tempers frayed -

"It was very tough, very intense at times, sometimes emotional and confrontational," Kerry told CNN late Thursday, just hours after clinching the outlines of a ground-breaking nuclear deal with Iran.

"It was a very intensive process because the stakes are very high and because there is a long history of not talking to each other, for 35 years we haven't talked to the Iranians directly like this," he said.

He admitted to the BBC that at one point during the rollercoaster eight-day talks, robbed of sleep as they haggled through the night, he had told Zarif he would walk away.

"I believe in diplomacy. I believe in the possibilities of trying to resolve things... War is the absence of diplomacy, or the failure of it," Kerry told ABC late Thursday.

But it is also testimony to Kerry's doggedness, and the way aides say has immersed himself in the complex technical details of nuclear energy, proliferation and Iran's atomic program.

He brought his own energy to the negotiations which over the past 18 months have involved a dedicated team led by the indefatigable Under Secretary Wendy Sherman backed by hundreds of US experts.

The State Department official admitted that at times even the unflappable Kerry lost patience during the ups and downs of the difficult Lausanne talks.

"Everybody was passionate. Everybody has a different style. The secretary is an incredibly patient man, very thoughtful and calm and I think as a result when he's had it, it has quite an impact because it's not his usual style."