Sen. John McCain said Monday he is still undecided on whether he will vote to confirm Rex Tillerson as secretary of state, but he is "barely" leaning toward a yes vote.

McCain has expressed concern over Tillerson's ties to Russian President Vladimir Putin, who McCain has described as "a thug, a bully, and a murderer."

Tillerson, as CEO of Exxon Mobil, was given Russia's Order of Friendship in 2013 and opposed sanctions against the country when his company was drilling for oil there. But he told wary Republicans and Democrats during his confirmation hearing last week he sees Russia as a threat and the Kremlin was wrong to annex the Crimea in 2014.

McCain has said President-elect Donald Trump should be given the benefit of the doubt on his nominations. The Arizona Republican told Fox News Channel's "The First 100 Days" he spoke with Tillerson again Monday, and some of his concerns have been satisfied.

"I haven't made up my mind completely," McCain said. "I just want him to have an understanding of the moral dimensions of Vladimir Putin, in particular, and the slaughter that he has inflicted, and the violation of solemn treaties when he annexed Crimea. The guy is a butcher."

Putin only understands strength, McCain said, adding he hopes Trump, who has said he would like to have a good relationship with the Russian leader, will not make the same mistake as the past three U.S. presidents who have tried to work with him.

George W. Bush said in 2001 he looked into Putin's eyes and could see his soul, finding him "very straightforward and trustworthy." Barack Obama in 2012 was caught on microphone telling outgoing Russian President Dmitry Medvedev to tell Putin to give him more "space" on missile defense issues and "after my election I have more flexibility."

But the only thing Russia understands is peace through strength, McCain told Fox News host Martha McCallum.

"That is what Ronald Reagan practiced, and that's what we got to go back to," McCain said.

"After your conversation with Rex Tillerson, would you say you are leaning toward supporting him?" McCallum asked.

"Barely, yes." McCain said, laughing.