In his remarks during confirmation hearing in the Senate, the retired Marine General said the US must be ready to confront Russian behavior in areas where the two countries cannot cooperate.
Donald Trump's pick for US Secretary of Defense General James Mattis and the nominee to head the CIA, Mike Pompeo portrayed Russia as a threat during their confirmation hearings in the US Senate on Thursday.
Retired Marine Corps General James Mattis said Russia, China and militants were presenting the biggest challenge to the US-led world order since World War Two, and called for Congress to lift spending caps undermining military readiness.
Asked about the main threats to US interests, he said: "I would consider the principle threats to start with Russia."
In his written remarks, Mattis said the US must be ready to confront Russian behaviour in areas where the two countries cannot cooperate, even as he backed Trump's bid to engage with Moscow.
"We engaged with Russia even during the darkest days of the Cold War, and I support the President-elect's desire to engage with Russia now," the retired Marine General said.
"At the same time, when we identify other areas where we cannot cooperate, we must confront Russia's behaviour, and defend ourselves if Russia chooses to act contrary to our interests."
"I'm all for engagement but we also have to recognise reality in what Russia is up to," Mattis said, adding there were a "decreasing number of areas" where the United States might cooperate with Moscow.
Later, Trump's nominee to head the CIA, Mike Pompeo, portrayed multiple challenges facing the US, from an aggressive Russia to a "disruptive" Iran to a China that he said is creating "real tensions."
Pompeo said that Russia is "asserting itself aggressively" by invading and occupying Ukraine, threatening Europe, and "doing nearly nothing" to destroy Islamic State (Daesh).
He said the CIA does not make policy on any country, adding, "it is a policy decision as to what to do with Russia, but it will be essential that the Agency provide policymakers with accurate intelligence and clear-eyed analysis of Russian activities."
Pompeo also said that he was "pretty sure" Russia was behind election hack. The nominee for CIA chief is a Republican member of the House of Representatives and a former US Army officer.