It is a "fair assumption" that Russian President Vladimir Putin was behind US election hacks, secretary of state nominee Rex Tillerson has said.
The former CEO of Exxon Mobil told his Senate confirmation hearing the intelligence report on Russian tampering "clearly is troubling".
His comments came after Senator Marco Rubio pressed Mr Tillerson to admit Mr Putin's role in the cyber-breach.
Mr Tillerson's reported good ties with Mr Putin have alarmed some in the US.
The 64-year-old former corporate titan, who has never worked in government, faced tough questions from the Senate Foreign Relations Committee on Wednesday.
In a heated exchange, Mr Rubio grilled him on whether intelligence reports about Russia's involvement in hacks of the US Democratic party were accurate and if Mr Putin had directed the attacks.
Mr Tillerson said he had no inside information on the detailed intelligence about Russia's hacking, but he had read the declassified US report released last week on the issue.
The Florida senator suggested that Mr Putin was responsible for war crimes because of Moscow's support for Syrian President Bashar Assad and bombing of Aleppo.
But the Texan multimillionaire told Mr Rubio he would not describe Mr Putin as a war criminal.
"I would not use that term," Mr Tillerson said. "Those are very, very serious charges to make and I'd want to have much more information before reaching that conclusion," he added.
The Florida senator - who was one of Mr Trump's rivals for the Republican nomination - said he had "serious concerns" about Mr Tillerson as America's top diplomat.
In other testimony: Mr Tillerson said:
- He disagrees with Mr Trump's remarks that more countries should have nuclear weapons
- He does not oppose the Trans-Pacific Partnership (TPP), a massive trade deal that Mr Trump has vowed to scrap his first day in office
- The 2015 nuclear deal with Iran should be given a "full review", though he did not call for it to be rejected outright
- The president should veto any bill that lifts the embargo on Cuba, and review whether Cuba should have been removed from the state sponsors of terrorism list
- The risk of climate change does exist and the consequences could be serious enough to take action, but "our abilities to predict that effect are very limited"
- China is a growing economic and military power and its "island-building in the South China Sea is an illegal taking of disputed areas without regard for international norms"
Tillerson in jeopardy? - Anthony Zurcher, BBC North America Reporter
It looks like secretary of state nominee Rex Tillerson has his work cut out for him if he wants to win the support of Florida Republican Senator Marco Rubio.
In a blistering 10 minutes of questioning, the former presidential candidate, who was once belittled by Donald Trump as "little Marco", peppered the president-elect's chosen man with a series of questions about his views on Russia.
Mr Tillerson's answers clearly didn't satisfy Mr Rubio, who said they were "discouraging", as he shook his head throughout the exchange.
Given the narrow majority Republicans have in the Senate, if Mr Rubio - and a handful of other Republicans - fail to back Mr Tillerson, his nomination could be on shaky ground.
In fact, the Florida senator, through his position on the confirmation committee, could join forces with Democrats to delay or even derail Mr Tillerson's bid to be secretary of state before it reaches a vote in the full Senate.
With Mr Trump's relations with Russia in the US political spotlight over the past few days, it's Mr Tillerson's nomination that may be in the greatest jeopardy.
While Mr Tillerson was grilled by senators in Washington DC, up in New York Mr Trump was rejecting claims that Russian intelligence agencies have compromising information about the president-elect.
In his first news conference in nearly six months on Wednesday, at Trump Tower, Mr Trump dismissed the allegations against him as "fake news" and "phony stuff" crafted by "sick people".
Russia has called the allegations "pulp fiction" and a "clear attempt to damage relations".
In his Senate statement, Mr Tillerson warned that Americans should be "clear-eyed about our relationship with Russia".
"Russia today poses a danger, but it is not unpredictable in advancing its own interests. It has invaded Ukraine, including the taking of Crimea, and supported Syrian forces that brutally violate the laws of war.
"Our Nato allies are right to be alarmed at a resurgent Russia," he continued, adding: "But it was in the absence of American leadership that this door was left open and unintended signals were sent."
It is Mr Tillerson's connections to Russia that have drawn the most flak in recent months.
He has forged multi-billion-dollar deals with Russia's state oil company, Rosneft, spoken out against international sanctions imposed on Moscow and in 2013 was awarded an Order of Friendship by the Kremlin.