Rex Tillerson, the ExxonMobil chief picked to head the US State Department, has built close ties with leaders around the globe, but most notably -- and controversially -- with Russia's Vladimir Putin.

Trained as an engineer, the silver-haired oilman is 64 and has never worked in government, but his global deal-making experience could be an asset in defending US interests.

His close relationship with Putin likely was key to President-elect Donald Trump's choice. Trump is keen to improve ties with Russia which soured greatly when Russia annexed Crimea in 2014.

But it also will be a key point of contention when Tillerson comes up for confirmation by the Senate, against the backdrop of US intelligence indicating that Russia interfered to try to sway the US election for Trump.

In a statement announcing the nomination, Trump said of Tillerson: "his relationships with leaders all over the world are second to none."

The ExxonMobil chief "has had more interactive time with Vladimir Putin than probably any other American with the exception of Henry Kissinger," said John Hamre of the Center for Strategic and International Studies (CSIS). Tillerson is a CSIS trustee.

And if Trump sees Tillerson as dynamic and able to get results, critics from Republican John McCain to environmentalists have a long list of concerns and doubts, including putting an oilman in charge of the US role in global climate change accords.

McCain has said that Tillerson's close ties to Putin were "a matter of concern."

"I'd have to examine it," he said, adding: "Vladimir Putin is a thug, bully and a murderer, and anybody else who describes him as anything else is lying."

- Russia's Order of Friendship -

Tillerson and Putin met in the 1990s when the oilman supervised an Exxon project on Sakhalin Island and strengthened their ties when Putin took power after Boris Yeltsin resigned in December 1999.

Their "friendship" was crowned by a historic agreement Exxon signed in 2011 with Russian public energy giant Rosneft to explore and drill in the Arctic and Siberia.

The deal, at first valued at $3.2 billion, could potentially generate a hefty $500 billion depending on oil discoveries -- but has been put on hold by Western sanctions against Russia.

Tillerson, who was awarded the Order of Friendship by Putin in 2013, lashed out against the sanctions at a shareholders meeting in 2014.

"We always encourage the people who are making those decisions to consider the very broad collateral damage of who are they really harming with sanctions," he said.

- Foreign policy goals? -

Born in Wichita Falls, Texas, Rex Tillerson has spent his entire career at Exxon, which he joined in 1975. Appointed CEO in 2006, he was due to retire in March.

His views on foreign policy are little known, aside from the fact he is a proponent of free trade.

Among the key issues awaiting him as secretary of state, he would oversee the Iranian nuclear deal. Trump has said he wants to review the 2015 accord struck between Iran and the United States, China, Russia, Germany, France and Britain.

He also will handle sanctions against Russia, rows with China and the protracted Syrian conflict.

His action on climate change will be closely scrutinized, after he resisted cutting investment in the search for new oil wells.

Several US states including New York, supported by environmental activists, are suing the oil giant for allegedly deceiving the public about the role of fossil fuels in global warming.

His nomination is "unfathomable," says environmental group 350.org.

"We can not let Mr. Trump name the world's largest oil company in charge of our international climate policy. Mr. Tillerson may be a friend of Mr. Putin, but he is not a friend of the planet," the NGO argued, offering an online a petition against his confirmation.

Tillerson did come out in favor of a carbon tax in 2009, which his predecessor Lee Raymond fought.

His position as a shareholder of Exxon, in which he holds $150 million in shares according to stock exchange documents, could pose a conflict of interest since his decisions as top US diplomat could influence the share price. And if sanctions on Russia were dropped, the ExxonMobil share value likely would soar.

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