US President-elect Donald Trump's picks to lead the Pentagon and CIA have taken a tough stance against Russia at their confirmation hearings, distancing themselves from the pre-election rhetoric of the future Oval Office boss.

Retired Marine General James Mattis, the defence secretary nominee, told senators on Thursday that Washington must be ready to confront Moscow when necessary.

Asked about the main threats to US interests, Mattis said: "I would consider the principle threats to start with Russia."

Mattis also said Russia, China and armed groups were presenting the biggest challenge to the US-led world order since World War II, accused Moscow of trying to break up NATO, 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 that there were a "decreasing number of areas" where the US might cooperate with its old Cold War foe.

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His remarks were the latest by one of Trump's cabinet picks to stand in sharp contrast to the views of their future boss, who has previously praised Russian President Vladimir Putin and pledged to improve ties with him.

"I have to say I don't remember a time where we've seen cabinet secretaries that seem to have such different views than the person they hope to work for: president-elect Donald Trump," Al Jazeera's Patty Culhane, reporting from Washington DC, said.

On Wednesday, Rex Tillerson, the secretary of state nominee, had also expressed views at odds with Trump on key foreign policy issues such as nuclear proliferation, trade deals, climate change and relations with Mexico.

Mattis said he wanted to meet with the new Trump national security team to "craft a strategy to confront Russia for what it's done", when questioned about the possibility of new US sanctions.

Confirmation hearings got under way on Thursday also for Mike Pompeo, Trump's choice to lead the CIA, amid tension between the president-elect and the agency Pompeo has been tapped to lead.

In his remarks to Senators, Pompeo also veered away from Trump, who wants to warm relations with Moscow.

He said the US faced one of the most "complicated" threat environments in decades, including an increasingly aggressive Russia "threatening Europe" and "doing nothing" to defeat the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in Syria.

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On other issues, Pompeo said Iran has become an "even more emboldened and disruptive player in the Middle East".

As CIA director, he said he would drop the opposition he has had as a politician to the 2015 nuclear deal between Iran and major powers.

Yet, Pompeo called the Iranians "professionals at cheating" and said he would work to improve US capability to detect violations of Tehran's commitment to curb its nuclear activities.

Trump this week furiously denounced intelligence officials for what he said were leaks to the media by intelligence agencies of a dossier that makes unverified compromising allegations about his contacts in Russia.

By contrast, Pompeo voiced strong support for the agency, saying he has seen Central Intelligence Agency staff "walk through fire".

He said he understood it would be a problem "if folks were afraid there would be political retribution" and promised "to have their backs at every single moment. You have my word I will do that."