Trump's defense secretary tells NATO allies they need to contribute more to their defense or US will 'moderate' its commitments

Defense Secretary Gen. Jim Mattis reportedly told NATO members Wednesday that they should step up their defense spending if they don't want the US to "moderate" its commitments to the alliance.

"I owe it to you all to give you clarity on the political reality in the United States, and to state the fair demand from my country's people in concrete terms," Mattis said during a closed-door meeting with NATO defense ministers, according to The Washington Post.

"America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to the alliance, each of your capitals needs to show its support for our common defense."

It's unclear how exactly the US would "moderate" its commitments to the alliance if NATO members refused to up their spending to meet the requirement that they spend at least 2% of their GDP on their own defense.

"No longer can the American taxpayer carry a disproportionate share of the defense of western values," Mattis reportedly said. "Americans cannot care more for your children's security than you do. Disregard for military readiness demonstrates a lack of respect for ourselves, for the alliance, and for the freedoms we inherited, which are now clearly threatened."

Mattis is echoing the sentiments of President Donald Trump, who during his presidential campaign and after called NATO "obsolete" and suggested that the US might not fulfill its commitments to the alliance if member countries refused to commit more money to their own defense.

Five of NATO's 28 member countries currently meet their requirement for spending 2% of their GDP on defense.

The Soufan Group, a strategic-security firm, noted last month that Trump's position on NATO has "generated concerns of potentially unprecedented changes in US foreign policy" toward Europe.

A note by the group said that "even the suggestion of the US taking a less robust role in leading or supporting NATO is without precedent." NATO receives significant support financially and militarily from the US.

Still, it's unclear how exactly Trump's views will translate to specific policy actions.

Secretary of State Rex Tillerson said during his confirmation hearing that NATO's Article 5 collective-action agreement is "inviolable" and that the US will stand by it. Trump has also that NATO was "very important to him."