Pentagon chief vows 'unshakeable commitment' to NATO: spokesperson

New US Secretary of Defense James Mattis reassured his British counterpart on Monday that Washington has an "unshakeable commitment" to NATO, despite President Donald Trump previously casting the military alliance as obsolete.

During a phone call with Michael Fallon on his first full day in office, Mattis "emphasised the United States' unshakeable commitment to NATO," Pentagon spokesperson Captain Jeff Davis said in a statement.

NATO, Russia, Merkel, Brexit: Trump unleashes broadsides on Europe

Ahead of his inauguration, Trump told two European newspapers he had long warned that NATO had "problems." "Number one, it was obsolete, because it was designed many, many years ago," he said, referring to its Cold War, post-World War II origins. "Number two, the countries aren't paying what they're supposed to pay."

Trump said in his inaugural address on Friday that the United States had "subsidised the armies of other countries," highlighting a common cause of friction in the 28-nation alliance.

In response to Trump's remarks, German Chancellor Angela Merkel warned that Europe has to take responsibility for itself.

British PM says Trump 'recognises importance of NATO'

During his phone call with Fallon, Mattis also "emphasised the United States and the United Kingdom will always enjoy a uniquely close relationship, reflected in our defense ties which are a bedrock of US security," according to his spokesperson.

British Prime Minister Theresa May will visit Washington on Friday. She will be the first foreign leader to meet with Trump since he took office last week.