WASHINGTON • US Secretary of Defence James Mattis has reassured Nato allies that Washington has an "unshakeable commitment" to the military alliance, despite President Donald Trump previously deriding the organisation as "obsolete".

The retired Marine Corps general spoke by telephone with Nato secretary-general Jens Stoltenberg and his British and Canadian counterparts on Monday, his first full day in office.

Mr Mattis "emphasised the United States' unshakeable commitment to Nato" in his call with British Defence Secretary Michael Fallon, Pentagon spokesman Jeff Davis said in a statement.

He "emphasised the United States and the United Kingdom will always enjoy a uniquely close relationship, reflected in our defence ties, which are a bedrock of US security", Mr Davis added.

British Prime Minister Theresa May will visit Washington on Friday, the first foreign leader to meet Mr Trump since he took office.

In his call with Mr Stoltenberg, Mr Mattis, who had previously served in a senior Nato post, discussed "the key role Nato plays in transatlantic security".

He "wanted to place the call on his first full day in office to reinforce the importance he places on the alliance", the statement said.

"The two leaders discussed the importance of our shared values", and Mr Mattis "emphasised that when looking for allies to help defend these values, the United States always starts with Europe".

In the phone call to Canadian Defence Minister Harjit Sajjan, the two men "reiterated the depth and breadth" of the US-Canada relationship as partners in the North American Aerospace Defence Command, "Nato allies and North American neighbours". They also "addressed the importance of North American defence ties among the US, Canada and Mexico", the statement said.

AGENCE FRANCE-PRESSE