Defense Secretary James Mattis on Wednesday delivered an ultimatum to NATO during his first meeting there as Pentagon chief: Pay more or the U.S. will “moderate its commitment.”

“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 the closed-door meeting, 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.”

Mattis’s comments come shortly after he called the alliance a “fundamental bedrock” for the U.S. in public remarks before the meeting.

NATO members have been shaken by President Trump repeatedly calling NATO “obsolete” during the campaign and questioning whether he would come to the defense of allies if they didn’t shoulder more of the cost of their defense.

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Just five of 28 members meet the alliance’s goal of spending 2 percent of their gross domestic product on defense: the U.S., the United Kingdom, Greece, Estonia and Poland.

On Tuesday during a press conference previewing the defense ministers’ meeting, NATO Secretary General Jens Stoltenberg said that defense spending would be a focus.

But Stoltenberg also said progress has been made over the last year to increase spending. In 2016, European members and Canada increased defense spending by 3.8 percent, amounting to an increase of about $10 billion, he said.

Mattis, though, said in the meeting that it’s now a “governmental reality” that the United States would lose patience with members being slow to meet the spending target.

“No longer can the American taxpayer carry a disproportionate share of the defense of western values,” Mattis 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.”