US Secretary of State nominee Rex Tillerson called US intelligence findings of Russian interference in the US presidential election "troubling" but said he has not yet seen classified information surrounding allegations that Russia intended to help President-elect Donald Trump.
Under combative questioning from Senator Marco Rubio, a 2016 GOP presidential candidate, Tillerson, a longtime oil executive, did not dispute the overall finding of Russian meddling - the extent and intent of which Trump has openly questioned.
Tillerson said he has not yet received his security clearance and thus has not received any classified briefings. But he said he had reviewed the unclassified report that US intelligence agencies released last week on Russian interference in the election. More detailed classified versions of that report were presented to Trump and to President Barack Obama.
"That report clearly is troubling and indicates that all of the actions you just described were undertaken," Tillerson said during his confirmation hearing before the Senate Foreign Relations Committee.
Tillerson pledged to separate his business background from his new role if confirmed as the country's top diplomat, as senators began grilling the ExxonMobil executive about his views and friendships with autocratic leaders.
Rubio, who had expressed scepticism about Tillerson's ties to Russia, came out swinging. He is the only Republican who has suggested he might oppose Tillerson.
Under questioning from Rubio, Tillerson refused to label Russian President Vladimir Putin as a war criminal.
"I would not use that term," Tillerson said.
Rubio then described Putin and the Russian military's behaviour in Aleppo, Syria, where they are accused of indiscriminately targeting civilians and causing mass casualties in that country's civil war.
"Those are very, very serious charges to make and I'd want to have much more information before reaching that conclusion," Tillerson said.
Rubio said he found that comment "discouraging".
The top Democrat on the panel, Senator Benjamin Cardin, asked Tillerson whether Russia has a legal claim to Crimea, the region of Ukraine it invaded and annexed in 2014.
"No, sir," Tillerson said. "That was a taking of territory that was not theirs."
Tillerson embraced what the incoming Trump administration calls an "America first" foreign policy and blamed some foreign-policy setbacks on what he called a failure or retreat of American leadership. He addressed concerns that Trump would back away from American commitments to the Nato alliance or be too deferential to Russia.
"Our Nato allies are right to be alarmed at a resurgent Russia. But it was in the absence of American leadership that this door was left open and unintended signals were sent," Tillerson said at the start of two planned days of confirmation hearings.
The 64-year-old Texan, recommended to Trump as a dark horse candidate but with no government experience, had his first chance to address concerns that the company he led, ExxonMobil, put profits ahead of human rights, environmental and policy concerns, and that he is too cosy with Russian President Vladimir Putin.
"We backtracked on commitments we made to allies," Tillerson said in apparent reference to the foreign policy of President Obama.
"We sent weak or mixed signals with 'red lines' that turned into green lights. We did not recognise that Russia does not think like we do."
Trump has called for cooperation with Russia on counterterrorism and other aims.
Tillerson faces tough opposition from Democrats on the Senate Foreign Relations Committee and may have an uphill path to confirmation because of the likely opposition of one or more Republicans.
Committee chairman Senator Bob Corker, warned: "It's going to be long", in part because Democrats have requested ample time to ask Tillerson questions and to review his record in public.
Tillerson comes to the committee well-prepared, Corker said, because he has met with every senator who is a member of the panel.
Democrats made the increasingly troubled US relationship with Moscow a dominant theme in the questioning. They also planned queries surrounding the oil giant's record on climate change.
Cardin asked Tillerson whether he believed US participation in international climate talks was something he favoured.
Tillerson, whose company has been in conflict with environmental groups, said, "It's important that the United States maintains its seat at the table about how to address the threat of climate change, which does require a global response. No one country is going to solve this on its own."
In his opening remarks, Tillerson did not mention concerns about his business history or relationship with Putin, with whom he has a long-standing relationship. A Trump transition office statement said that Tillerson's "relationships in some countries extend back multiple administrations and regimes" and that "in some cases he has more institutional knowledge than the current government."
"We are the only global superpower with the means and the moral compass capable of shaping the world for good," Tillerson said.
"In recent decades, we have cast American leadership into doubt. In some instances, we have withdrawn from the world. In others, we have intervened with good intentions but did not achieve the stability and global security we sought."
In apparent reference to commitments by Nato members to increase their defence spending, he said that "we cannot look the other way at allies who do not meet their obligations."
"We cannot afford to ignore violations of international accords, as we have done with Iran," he said. "We cannot continue to accept empty promises like the ones China has made to pressure North Korea to reform."
Tillerson criticised China for violating "international norms" in the South China Sea, contravening global economic agreements, stealing US intellectual property and for its aggression "in the digital realm".
He made no mention of the intelligence community's charges of Russian election interference, and appeared to blame the Obama administration for failing to understand Moscow's objectives.
"Russia today poses a danger," he said, "but it is not unpredictable in advancing its own interests."
It is not clear that any Democrats on the committee will vote for the nominee. Both Senator John McCain and Senator Lindsey Graham, have indicated reluctance to vote for Tillerson. That could endanger the nomination in the Senate, which Republicans hold with 52 seats.