Comments come just ahead of British Prime Minister Theresa May's visit to Washington on Friday and German Chancellor Angela Merkel's warning that Europe has to take responsibility for itself.
Although US President Donald Trump has in the past defined NATO as obsolete, newly-appointed US Secretary of Defense James Mattis reassured his British counterpart on Monday that Washington has an "unshakeable commitment" to the military alliance.
During a phone call with UK's Secretary of State for Defence Michael Fallon, Mattis "emphasised the United States' unshakeable commitment to NATO," Pentagon spokesman Captain Jeff Davis said in a statement.
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."
US President Donald Trump had also said that while the military alliance was still very important to him, the North Atlantic Treaty Organization was nonetheless obsolete because it had not defended against terror attacks.
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 defence ties which are a bedrock of US security," according to his spokesman.
Mattis on his first full day in office also spoke with NATO Secretary-General Jens Stoltenberg to emphasise the importance the US places on the military alliance.
"The two leaders discussed the importance of our shared values, and the secretary emphasised that when looking for allies to help defend these values, the United States always starts with Europe," Pentagon spokesman Captain Jeff Davis said in a statement regarding the call.
"Both pledged to consult in the months to come and look forward to meeting in person during the NATO Defence Ministerial in February," Davis said.