Defense Secretary Jim Mattis issued an ultimatum Wednesday to allies in the North Atlantic Treaty Organization, warning that if they do not boost their defense spending to goals set by the alliance, the United States may alter its relationship with them.

“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. “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.”

The statements came during a closed-doors meeting with defense ministers from other NATO countries and were provided to reporters traveling with the defense secretary to Brussels. It marks an escalation in Washington’s long-running frustration that many NATO countries do not spend at least 2 percent of their gross domestic product as they have pledged. President Trump often made that point during his upstart run for the White House, at various times calling the alliance “obsolete” while grousing that its 28 members need to pay “their fair share.”