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 "emphasized the United States' unshakeable commitment to NATO," Pentagon spokesman Captain Jeff Davis said in a statement.

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.