Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) said Saturday he has concerns about the possibility of Exxon Mobil CEO Rex Tillerson being nominated for secretary of State, given his ties to Russian President Vladimir Putin.

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“I don’t know what Mr. Tillerson’s relationship with Vladimir Putin was, but I’ll tell you it is a matter of concern to me,” McCain said in an interview with Fox News.

NBC News reported earlier on Saturday that President-elect Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE had settled on Tillerson to head the State Department, though the transition team has not confirmed the report.

McCain is the first Republican to raise questions about the potential nomination, and Tillerson could fail to make it through the confirmation process if just three GOP senators join a united Democratic opposition.

“I’d have to examine it,” McCain said of the potential nomination. “You want to give the president of the United States the benefit of the doubt because the people have spoken. But Vladimir Putin is a thug, bully and a murderer, and anybody else who describes him as anything else is lying.”

Former top McCain aide Mark Salter predicted that Tillerson would have a hard time getting through the confirmation process.

“Tillerson would sell out NATO for Sakhalin oil and his pal, Vlad. Should be a rough confirmation hearing, and a no vote on the Senate floor,” Salter tweeted Friday.

Tillerson negotiated an energy partnership with Putin in 2011 that the Russian president said would be worth as much as $500 billion. The next year, the Exxon Mobile CEO received the Russian Order of Friendship from the Kremlin, one of the highest honors that Russia bestows on foreigners.