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"