Donald Trump's pick for defence secretary has put Russia at the top of a list of threats to US interests and told Congress that America must be ready to confront it where necessary, even as he backed Trump's bid to engage with Moscow.
Retired Marine General James Mattis said Russia, China and Islamist 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.
"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 co-operate with Moscow.
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Asked about the main threats to US interests, Mattis said: "I would consider the principal threats to start with Russia."
Trump on Wednesday acknowledged that Russia likely hacked the Democratic National Committee and emails of top Democrats during the 2016 presidential election campaign.
Mattis cited Russian involvement in hacking and information warfare among the challenges posed by Moscow.
Others include treaty violations, using tactics short of open war to destabilise other countries and "alarming messages from Moscow regarding the use of nuclear weapons."
Mattis also singled out China for its activities in the South China Sea, where it has been building man-made islands with anti-aircraft and anti-missile batteries.
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Mattis said China was part of mounting assault on global stability, and the relationship with Beijing needed to be carefully managed.
"I think it (the world order) is under the biggest attack since World War Two, sir, and that is from Russia, from terrorist groups, and with what China is doing in the South China Sea," he said.
Mattis, who retired from the military in 2013, is technically ineligible for the job since he has not been a civilian for at least seven years.
That means Congress would need to grant him a waiver, something it has not done since 1950.
After Mattis' testimony, the waiver cleared its first hurdle in Congress when the Senate Armed Services Committee approved it in a 24-3 vote.
In his opening statement, Mattis said he can lead the military as a civilian, even after a 44-year military career.
"I recognise my potential civilian role differs in essence and in substance from my former role in uniform," Mattis said.
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He said the US strategy to retake Islamic State's de facto capital in the Syrian city of Raqqa would to be reviewed and potentially re-energised.
Mattis was also critical of Iran's influence in Iraq, Syria, and Yemen.
"Iranian malign influence in the region is growing. Iran is the biggest destabilizing force in the Middle East and its policies are contrary to our interests," Mattis said.
On Iraq, where a US-led coalition is backing Iraqi troops battling Islamic State, Mattis said the main goal should be to ensure "that it does not become a rump state of the regime in Tehran."
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