On the first day of his confirmation hearings, Secretary of State nominee and former Exxon Mobil chief executive Rex Tillerson was given a serious grilling by Marco Rubio.

The Florida senator, who was beaten to the Republican nomination by President-elect Donald Trump, pressed Tillerson on his close links to Vladimir Putin and his thoughts on the Russian President's past actions.

A crucial point came when Rubio pressed Tillerson on whether he thought Putin was a war criminal. He then referred to Putin/Russia's involvement in the recent civilian bombings in Aleppo, and his previous involvement in the deaths of tens of thousands of Chechens in the city of Grozny in 1999.

The exchange is notable, in part, because it's a Republican senator being critical of a Republican president's cabinet pick. The White House recently accused the GOP of rubber stamping Trump's cabinet picks and trying to force them through quickly.

Tillerson's background

Tillerson has been under the microscope for his close ties to Russia during his time as the head of one of the world's biggest oil companies.

In 2013, Putin presented him with the Order of Friendship, one of the highest honors bestowed upon foreign citizens, and his apparently cozy links with the eastern nation has prompted concern with some politicians in Washington.

But while he stopped short of calling Putin a war criminal, Tillerson did say he would adopt a tough line on Russia. The Chicago Tribune reported he called Russia a "danger" to the United States and said he'd have recommended a "muscular" response to the country's annexation of Ukraine's Crimea region in 2014.

"If Russia acts with force," Tillerson said, "they require a proportional show of force."

Donald Trump himself was questioned on his Russian ties during a press conference on Wednesday, and said it should be seen as an "asset, not a liability" if he had a good relationship with Putin.

Tillerson's confirmation hearing will continue Thursday.