Story highlights Secretary of Defense James Mattis called three key NATO partners on first day to "reinforce the importance" of the alliance

Mattis spoke with NATO secretary general and counterparts from UK and Canada

Washington (CNN) Newly confirmed Defense Secretary James Mattis sent a strong signal of support to NATO on Monday, reaching out to three critical alliance partners and saying the US had an "'unshakeable commitment to NATO."

Mattis -- whose embrace of NATO differs from comments made by President Donald Trump -- made his first official calls to Secretary General of NATO Jens Stoltenberg and to his counterparts from two key NATO allies, UK Defense Secretary Michael Fallon and Canadian Minister of Defense Harjit Sajjan.

According to the Pentagon's official readout, Mattis in his conversation with Fallon "emphasized the United States' unshakeable commitment to NATO."

White House press secretary Sean Spicer was asked about Mattis' comments at the press briefing Tuesday.

"The President is very clear that as it's structured now, in terms of the output of NATO, he doesn't feel as though it's doing what its mission was set up to do or that it's being particularly effective," Spicer said.

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