Sen. John McCainJohn McCainBush biographer: Trump has moved the goalpost for civilized society White House to pressure McConnell on ObamaCare McCain: Trump needs to state difference between bigots and those fighting hate 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 TrumpDonald John TrumpAssange meets U.S. congressman, vows to prove Russia did not leak him documents A history lesson on the Confederacy for President Trump GOP senator: Trump hasn't 'changed much' since campaign 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.

The energy deal was put on hold when the U.S. levied sanctions against Russia for annexing Crimea. Reuters reported earlier this year that Exxon vowed to resume the agreement once sanctions are rolled back, a process Tillerson would be heavily involved in as secretary of State.