McCain, who is chairman of the US Senate Armed Services Committee, also called President Putin "a thug and a murderer". Photograph: (Getty)
He, and many Democrats and Republicans, are wary of Tillerson's personal and business connections to Vladimir Putin
U.S. President-elect Donald Trump's choice for Secretary of State, Rex Tillerson, will have to explain his relationship with Russian leader Vladimir Putin at his confirmation hearing, senior Republican senator John McCain said on Thursday.
McCain, who is chairman of the US Senate Armed Services Committee, also called President Putin "a thug and a murderer".
Speaking on a tour of the Baltic states, where there is widespread concern that Moscow wants to reassert its power across the region, McCain said he had concerns about Tillerson, who opposed sanctions on Russia after its annexation of Crimea.
"I and several of my colleagues have concerns about Mr Tillerson, and some of his past activities, specifically his relationship with Vladimir Putin," McCain told reporters.
"I have concerns but at the same time I'm certain we will give Mr Tillerson an opportunity to make his case about why he is qualified to be Secretary of State."
Exxon Mobil Chief Executive Tillerson could face a rocky confirmation process, given worries among both Democrats and Republicans about his connections with Russia and possible conflicts of interest related to his business ties.
Tillerson owns approximately $218 million in ExxonMobil stock, and a pension worth about $70 million, the Washington Post reports.
When Obama administation slapped sanctions on Russia after Putin annexed Crimea, Tillerson voiced opposition--Exxon had just discovered an enormous reserve of oil in the Kara Sea, and Obama's sanctions prevented the company from making an enormous profit.
Assuming the senate confirms the appointment and he does become Trump's secretary of state, Tillerson, a man who personally received the "Russian Order of Friendship award" from Vladimir Putin himself, would be in a position to decide whether or not to continue sanctions against Putin's Russia.
Relations between Moscow and Washington have deteriorated following Russia's annexation of the Crimea in 2014, with the two nations also at loggerheads over Syria.
The Obama administration also plans a series of retaliatory measures against Russia for hacking into US political institutions and individuals and leaking information.
Trump, who before he won the election complained for weeks that it was rigged and when pressed on the issue refused to say that he would accept its result, has now said that the United States should move on from the controversy.
McCain, however, said the United States needed to reassure voters there and abroad that elections would not be influenced by foreign powers.
"I agree with president elect that we need to get on of our lives - without having elections being affected by any outside influence, especially Vladimir Putin, who is a thug and a murderer," he said.
Fellow Republican senator Lindsey Graham, who is travelling with McCain, called for sanctions against Russia to be widened to target areas like energy and to be directly aimed at Putin.
"I think the sanctions need to go beyond what it is today, they need to name Putin as an individual and his inner circle because nothing happens in Russia without his knowledge and approval," Graham said.
Graham has also stated that "99 per cent" of US Senators believe Russia interfered in the election. He never named the one per cent who apparently disagrees.
(With inputs from Reuters)