US Secretary of Defense James Mattis has reportedly spoken with Nato Secretary-General Jens Stoltenberg on Monday (23 January) to highlight the importance he places on the 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.

More from IBTimes UK

Throughout President Donald Trump's campaign for the White House, the Republican showed disdain for Nato, labelling it as "obsolete". However, he seems to have softened his stance in recent weeks.

He also suggested that the US might not come to the assistance of a Nato ally under attack if they were not contributing enough financially to the military costs.

Following his remarks however, Stoltenberg warned Trump that "going it alone" is not an option for Europe or the US.

"Going it alone is not an option, either for Europe or for the US. We face the greatest challenges to our security in a generation," he said in the Observer.

"This is no time to question the value of the partnership between Europe and the US.

"The only time Nato has invoked its self-defence clause, that an attack on one is an attack on all, was in support of the US after the 9/11 terrorist attacks. This was more than just a symbol.

"Nato went on to take charge of the operation in Afghanistan. Hundreds of thousands of European soldiers have served in Afghanistan since. And more than 1,000 have paid the ultimate price in an operation that is a direct response to an attack against the US."

In recent months Trump seemed to reverse his stance on Nato, saying that he will work closely with allies to defeat Islamic State (Isis).

Speaking before he won the US election, Trump said: "I had previously said that Nato was obsolete because it failed to deal adequately with terrorism; since my comments they have changed their policy and now have a new division focused on terror threats," adding that the apparent development is "very, very good".

In his statement regarding the call between Mattis and Stoltenberg, Davis added: "Both pledged to consult in the months to come and look forward to meeting in person during the Nato Defense Ministerial in February."

Davis also confirmed Mattis on Monday spoke with Britain's Secretary of Defence, Michael Fallon.

According to Davis, Mattis thanked Fallon for the UK's commitment of 2% of GDP to defence and contributions to international security.

NATO