Tillerson, 64, got his start as a production engineer at Exxon in 1975 and has worked there ever since, running business units in Yemen, Thailand and Russia before being named chief executive in 2006. He was expected to retire next year.

Critics suggested that if President-elect Donald Trump were to choose Tillerson – as a source familiar with the situation said he was expected to do – it would continue a trend of selecting some aides who may favour a softer line toward Moscow.

Tillerson’s emergence as the front-runner for the post has raised concern among Republican and Democratic senators over his ties to Russia.

“It’s a matter of concern to me that he has such a close personal relationship with Vladimir Putin and obviously they’ve done enormous deals together. That would colour his approach to Vladimir Putin and the Russian threat,” Republican Senator John McCain told CBS.

McCain added that Tillerson would, nonetheless, get a fair confirmation hearing.

Republican Senator Marco Rubio, a former Republican presidential rival to Trump, was even more forthright.

“Being a ‘friend of Vladimir’ is not an attribute I am hoping for from a #SecretaryOfState,” Rubio said on his Twitter account.

Many US officials are worried by Russia’s increasingly aggressive behaviour. It annexed Crimea from Ukraine in 2014, has supported Syrian President Bashar al-Assad in the Syrian civil war and is accused of interfering in US domestic politics.

US intelligence analysts have concluded that Russia intervened in the 2016 election to help Trump defeat Hillary Clinton, and not just to undermine confidence in the US electoral system, a senior US official said.

In his role at Exxon Tillerson maintained close ties with Putin and opposed US sanctions against Russia for its incursion into Crimea.

Trump praised Tillerson, saying on his Twitter account on Saturday: “Whether I choose him or not for “State”- Rex Tillerson, the Chairman & CEO of ExxonMobil, is a world class player and dealmaker. Stay tuned!”

Reince Priebus, the Republican National Committee chairman who has been tapped to serve as White House chief of Staff, praised Tillerson’s relationship with Putin.

“… the fact that he actually has a relationship with people like Vladimir Putin and others across the globe is something that … we shouldn’t be embarrassed by it. It’s something that I think could be a huge advantage to the United States,” Priebus said on ABC This Week.

However, Senator Robert Menendez of New Jersey, a senior Democratic member of the Senate Foreign Relations Committee that would weigh Tillerson’s nomination, was unsparing in his criticism of the possible appointment.

“Reports that Rex Tillerson could be nominated to be our nation’s top diplomat (are) alarming and absurd,” he said. “With Rex Tillerson as our secretary of state the Trump administration would be guaranteeing Russia has a willing accomplice in the president’s cabinet guiding our nation’s foreign policy.”

Tillerson would be one of the few people selected for major roles in the Trump administration to believe that human activity causes climate change.

After Trump’s election, Exxon came out in support of the Paris Climate Agreement. It has also advocated for a carbon tax and internally factors in a theoretical price on carbon as it weighs manufacturing and exploration costs of projects.

But some environmental groups are alarmed at the prospect of Exxon’s CEO as the country’s top diplomat.

Exxon is under investigation by the New York Attorney General’s Office for allegedly misleading investors, regulators and the public on what it knew about global warming.

“Donald Trump appears intent to undo a century of environmental and social progress and return America to the age of robber barons and corporate trusts,” said Carroll Muffett, president of the Center for International Environmental Law.

“Who better to turn to than Exxon, the granddaddy of them all?”

– Reuters