New US Secretary of Defense James Mattis reassured his British counterpart on Monday (Jan 23) that Washington has an “unshakeable commitment” to Nato, despite President Donald Trump previously casting the military alliance as obsolete.

During a phone call with Mr Michael Fallon on his first full day in office, Gen Mattis “emphasised the United States’ unshakeable commitment to Nato,” Pentagon spokesman Captain Jeff Davis said in a statement.

Ahead of his inauguration, Mr 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.”

Mr Trump said in his inaugural address last 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 Mr Trump’s remarks, German Chancellor Angela Merkel warned that Europe has to take responsibility for itself.

During his phone call with Mr Fallon, Gen Mattis also “emphasized 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 spokesman.

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