The former US Marine Corps general made the comments after he met with fellow defence ministers on Wednesday in Brussels.
But the politician did make reference to “sharing the necessary costs to defend freedom” after President Trump blasting Nato members during his campaign trail for failing to pay into the alliance.
Mr Mattis said: “The alliance remains a fundamental bedrock for the United States and the trans-Atlantic community, bonded as we are together.
“As President Trump has stated, he has strong support for Nato and Nato is in the midst of transformation – it has always been adapting to security challenges, this is nothing new. Perhaps the pace of change has certainly picked up a bit but this is something we can deal with.”
James Mattis has spoken out in support of Nato despite Donald Trump's previous criticism
The alliance remains a fundamental bedrock for the United States and the trans-Atlantic community, bonded as we are together
He continued: “It’s absolutely appropriate the European Minister of Defence said last week ‘it’s a fair demand that all who benefit from the best defence in the world carry their proportionate share of the necessary costs to defend freedom and we should never forget ultimately it is freedom that we defend here at Nato.”
Mr Mattis said he was “confident” the military alliance would continue to adapt to “changing circumstances".
“We’ve done so in the past, there’s every reason for confidence that we will move out purposefully once again," he added.
11 things you need to know NATO Tue, February 14, 2017
NATO – The North Atlantic Treaty Organization is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4th April 1949. The organization constitutes a system of collective defence whereby its member states agree to mutual defence in response to an attack by any external party.
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Every member country, no matter how large or small, has an equal say in discussions and decisions. Photo shows: Signing the North Atlantic Treaty which marked the beginning of NATO, 1949.
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Donald Trump has been critical of Nato members not contributing enough to the military alliance
In a joint interview with The Times and German newspaper Bild last month, President Trump reiterated his scepticism around Nato with the US contributing 70 per cent of its defence budget.
Mr Trump said: "I said a long time ago that Nato had problems. Number one it was obsolete, because it was designed many, many years ago.
“Number two the countries aren’t paying what they’re supposed to pay.
“We’re supposed to protect countries, but a lot of these countries aren’t paying what they’re supposed to be paying, which I think is very unfair to the United States.”
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