Two of Donald Trump’s cabinet nominees have diverged from their commander in chief’s stance on Russia, warning of the potential threat it poses for United States defence and cyber security.

Mr Trump’s incoming secretary of defence James ‘Mad Dog’ Mattis said on Thursday (US time) the world order was “under the biggest attack since World War II”, with Russia topping the list of threats to US interests.

Meanwhile, incoming CIA director Mike Pompeo described Russia as a “sophisticated adversary” in the cyber realm.

Retired general Mattis told the Senate Armed Services Committee considering his nomination that Russia, China and Islamist militants were presenting the biggest challenges to the United States.

He also accused Russian President Vladimir Putin of trying to break the NATO alliance.

“I would consider the principal threats to start with Russia,” Mr Mattis, who is yet to be confirmed in the position, said.

“Russia is raising grave concerns on several fronts and China is shredding trust along its periphery.

“Right now, the most important thing is that we recognise the reality of what we deal with with Mr Putin, and we recognise that he is trying to break the North Atlantic alliance.

“I’m all for engagement but we also have to recognise reality in what Russia is up to.”

He said the US must take the “diplomatic, economic, military and the alliance steps, working with our allies, to defend ourselves where we must.

“My view is that nations with allies thrive and nations without allies don’t,” he said.

While Mr Mattis’s remarks at his Senate confirmation hearing appeared set to clinch the panel’s backing for his nomination as secretary of defence, they provided a stark contrast to his future commander-in-chief’s stance.

Mr Trump has previously dismissed claims Russia hacked the US election as “ridiculous”, accusing the intelligence agencies of peddling “fake news” in relation to Russia.

He has also voiced criticisms of NATO, which General Mattis described as “the most successful military alliance probably in modern history, maybe ever”.

Russia-wary politicians hope Mr Mattis can temper Mr Trump’s stated desire to partner with Moscow.

New CIA director defies Trump

Russia dominated Thursday’s Senate confirmation hearings, with Mr Trump’s choice to lead the CIA also defying the president-elect on several occasions.

Trump’s nominated director of the CIA, Mike Pompeo. Photo: Getty

Incoming CIA director Mr Pompeo was at odds with his would-be boss, embracing the intelligence report on the Russian breaches compiled earlier this month.

“With respect to this report in particular, it’s pretty clear about what took place here, about Russian involvement in efforts to hack information and to have an impact on American democracy,” he said.

“This was an aggressive action taken by senior leadership inside of Russia.”

Mr Pompeo listed Russia, as well as China, as “sophisticated adversaries” in the cyber realm, saying “hackers are all taking advantage of this new borderless environment” and “the CIA must continue to be at the forefront of this issue”.

The investigation will look into FBI director James Comey’s role in the Clinton email saga. Photo: Getty

FBI investigation into Clinton email probe

Meanwhile, a US government watchdog plans to examine whether the FBI followed proper procedures in its probe of Democratic presidential candidate Hillary Clinton’s use of a private email server.

The announcement follows outcry from Democrats who say Ms Clinton’s election loss to Mr Trump was in part due to Mr Comey bringing Ms Clinton’s emails back into the public spotlight less than two weeks before the vote.

The Justice Department’s Office of Inspector General said its probe would focus in part on decisions leading up to public communications by FBI director James Comey regarding Ms Clinton’s investigation, and whether underlying investigative decisions may have been based on “improper considerations.”