LONDON (Reuters) - Iranian Foreign Minister Mohammad Javad Zarif used charm and knowledge of the West to help strike the 2015 nuclear deal with world powers. But his smooth diplomacy masked deep suspicions of the United States, which pulled out of the agreement on Tuesday.

FILE PHOTO: FILE PHOTO: Iranian Foreign Minister Mohammad Javad Zarif attends a meeting in Moscow, Russia April 28, 2018. REUTERS/Grigory Dukor/File Photo/File Photo

“You should always smile in diplomacy,” he wrote in his 2013 memoir “The Ambassador”. “But you should never forget you are talking to an enemy.”

President Donald Trump’s rejection of the agreement is likely to be a major setback for Iran, and for Zarif, regarded by many as Tehran’s most talented diplomat since the 1979 Islamic Revolution.

The 2015 deal, which lifted international sanctions in return for curbs on Iran’s nuclear program, was Zarif’ crowning achievement to date.

Trump said he would reimpose U.S. economic sanctions on Iran to undermine “a horrible one-sided deal that should have never, ever been made.”

That drew fierce criticism from Iranian officials.

Members of parliament burned an American flag and a copy of the Iran deal - the Joint Comprehensive Plan of Action (JCPOA) - as a session of parliament began. They also chanted “Death to America”, according to the Iranian Students’ News Agency.

Zarif now faces uncertainty after years of complex, exhausting negotiations with Western powers. As the public face of the Iran deal, he can expect intense criticism from hawks.

PRESSURE FROM HARDLINERS

Some hardliners threatened Zarif with bodily harm after the deal was signed. These critics will be emboldened by Trump’s actions and might put pressure on Zarif to resign.

But the Islamic Republic might still need his diplomatic skills to try and salvage the deal.

Zarif’s familiarity with Western culture helped him forge close working relationships with American officials, who have long memories of the occupation of the U.S. embassy in Tehran, in which hardline students held diplomats hostage for 444 days.

He lived in the United States, from the age of 17 as a student in San Francisco and Denver, and subsequently as a diplomat to the United Nations in New York, where he served as Iranian ambassador from 2002 to 2007.

Zarif developed direct contacts with U.S. officials, despite this being a political taboo back in Iran, which served him well in the 1990s when he was involved in negotiations to free U.S. hostages held by the pro-Iranian Hezbollah group in Lebanon.

Mastery of English helped him build a rapport with foreign diplomats, particularly the then U.S. Secretary of State John Kerry, with whom he was on first-name terms during the heated and intensive nuclear talks.

“Unlike most of his predecessors, he actually has theoretical knowledge and practical experience in international relations,” said Ali Vaez, Iran Project Director at the Crisis Group think-tank.

Zarif’s email exchanges with Kerry at critical moments, including during the arrest of 10 U.S. marines in Iranian waters in 2016, defused tensions between the two countries that have had no political ties since 1980.

However, this direct line between Tehran and Washington was lost when Trump entered the White House. Zarif, smooth and reassuring during negotiations, also has a tough side.

“Trump’s ignorant hate speech belongs in medieval times - not the 21st Century UN - unworthy of a reply,” Zarif tweeted a few hours after Trump’s first speech at the UN General Assembly in which he called the nuclear deal an “embarrassment” and “one of the worst and most one-sided transactions the United States has ever entered into”.

In March, ridiculing the meeting between Trump and Saudi Arabia’s powerful Crown Prince Mohammed bin Salman and their discussion on Saudi purchases of U.S. military hardware, Zarif tweeted: “Some are exuberant to milk, and others even happier about being milked.”

DIVISIVE FIGURE

Zarif’s seeming ease with Western ways has made him a divisive figure in Iran, and hardliners have lined up to lambaste him for speaking so directly to the Islamic Republic’s enemies.

Secretly filmed footage emerged in May 2015 of Zarif arguing furiously in a closed session of parliament with a lawmaker who branded him a traitor.

“Zarif has spent almost all his diplomatic life outside Iran and has a good grip of international affairs - but this is his weak point, too,” said Hossein Rassam, former Iran adviser to Britain’s Foreign Office.

Some hardliners have already called Zarif a coward for studying in the United States during the 1980s, rather than defending his country in the 1980-88 war with Iraq, in which Saddam Hussein had Western and Gulf Arab support.

U.S. Senator Tom Cotton, a Republican opposed to the nuclear deal, copied the tactic by tweeting at Zarif in 2015: “you hid in U.S. during Iran-Iraq war while peasants & kids were marched to die”.

But Zarif responded by congratulating Cotton on the birth of his son, adding: “Serious diplomacy, not macho personal smear, is what we need.”

“I have known Javad Zarif for the past three decades and attest to his genuine belief in ‘Zero Tension’ in foreign relations,” Hossein Mousavian, a former nuclear negotiator for Iran, told Reuters.

While the foundation of the nuclear deal that Zarif build with meticulous care is shaking, the Iranian diplomat has not lost hope.

“I’ll spearhead a diplomatic effort to examine whether remaining JCPOA (nuclear deal) participants can ensure its full benefits for Iran. Outcome will determine our response,” he tweeted.