Donald Trump’s incoming Defence Secretary has said world order is “under the biggest attack since World War II”.

General James Mattis claimed that Vladimir Putin was trying to “break the Northern Atlantic alliance” and said the US needed to take steps to defend itself, at his Senate confirmation hearing.

The retired general was also asked about the nuclear arms deal with Iran, and he said that although it was the “biggest destabilising force in the Middle East”, that the US had made a deal and should stick to its word.

The two statements provide a stark contrast to his future boss, Mr Trump, who dismissed the claims about the Russian hack on the US as “ridiculous” and accused the intelligence agencies of peddling fake news related to Russia.

While Mr Trump has voiced criticisms of Nato, General Mattis described it as “the most successful military alliance probably in modern history, maybe ever”.

Mr Trump also vowed throughout his presidential campaign to rip up the Iran deal that was signed under President Barack Obama in 2015. At other times, Mr Trump pledged to renegotiate the deal, or clamp down on it so hard that it could collapse under the weight.

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General Mattis responded that the US needed to adopt a strategy to “checkmate Iran’s goal for regional hegemony”.

“When America gives its word we have to live up to it and work with our allies,” he said, implying the US should not abandon the agreement.

The general was asked about his recent opposition to women serving in combat roles in the US army. He said in 2015 that it would lead to “reduced standards” and that men and women, serving alongside each other, could not be expected to “act like little saints”.

In the hearing, however, when asked about the issue by Democratic senator Claire McCaskill, he said, “I have no plan to oppose women in any aspect of our military.”

As long as applicants meet the expected standards, he said, “that’s the end of the discussion”.

Current Defence Secretary Ash Carter opened all areas of the military to women in January last year.

General Mattis also came under scrutiny for making a comment at a veteran conference that it was “a lot of fun to shoot some people”.

New York Senator Kirsten Gillibrand asked him directly whether he believed openly gay people should not serve in the military, and he responded that the army should be as lethal as possible and that the army was a touchstone for patriotism.

She then asked whether he believed LGBT members would undermine the lethality of the army.