The U.S. nominee for secretary of state and former Exxon Mobil CEO Rex Tillerson must address his close ties to Russia, a Senate Democrat said.

Tillerson and Exxon this week parted ways in an effort to address potential conflicts of interest between his financial ties to one of the biggest oil companies in the world and becoming the nation's top diplomat. Having never served in public office, the transition team said Tillerson is qualified based on his ability to navigate geopolitical issues as a businessman.

U.S. Sen. Chris Coons, D-Del., a member of the Senate Foreign Relations Committee, said during an appearance on MSNBC he was concerned by Tillerson's close ties to Russia.

Tillerson worked closely with Russian oil company Rosneft, a target of U.S. sanctions, and received distinguished awards from Russian President Vladimir Putin. Coons said he was concerned because protecting shareholder value and the interests of the American people are two very different things.

"We talked at some length about whether he was willing to embrace the use of sanctions if they are strong, if they're multilateral, if they're enforced effectively for the advancement of human rights and American interests," Coons said. "He was generally positive in his response on that."

Arizona Sen. John McCain, a Republican, was less cautious, telling reporters he'd support Tillerson's nomination when "pigs fly," though an aide to the former presidential candidate later walked back that comment.

A profile of Donald Trump's Cabinet picks by the Brookings Institution said Tillerson was endorsed by top former security strategists, but his "personal relationship" with Putin may make for problems during the confirmation process.

On the president-elect's use of Twitter as his means of issuing policy statements, the Delaware senator was less nuanced.

"I raised with Mr. Tillerson my grave concerns that if President Trump doesn't rely on his secretary of state [and] secretary of defense, and conducts his own foreign policy in 3 a.m. Twitter wars, that we may end up in a real war," he said.