U.S. President Donald Trump's defence secretary warned NATO allies on Wednesday that they must honour military spending pledges to ensure the United States does not "moderate" support for the alliance.
Jim Mattis, on his debut trip to Brussels as Pentagon chief, also accused some nations of ignoring threats, including from Russia.
"America cannot care more for your children's future security than you do," Mattis said in closed-door remarks to NATO defence ministers, according to prepared remarks provided to reporters.
The comments represented some of the strongest criticism in memory of allies who have failed to reach defence spending goals.
Mattis said investing in common defence was vital, given the challenges that have emerged since 2014, when Russia annexed Crimea from Ukraine and Islamic State emerged as a threat on NATO member Turkey's southern border.
Trump was sharply critical of NATO during his election campaign, making European allies nervous by calling the alliance obsolete and praising Russian President Vladimir Putin.
But Trump has since said he strongly supported NATO, comments echoed by Mattis at NATO headquarters. Mattis called NATO "the most successful and powerful military alliance in modern history".
Mattis, however, suggested U.S. support should not be seen as a given.
"I owe it to you 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 this Alliance, each of your capitals needs to show support for our common defence."
Mattis stopped short of issuing an explicit ultimatum or say how the United States might moderate its support. The United States for years has called for European allies to spend 2 percent of economic output on defence, he said.
But he added that NATO allies must show progress in 2017 and adopt a plan, with target dates, toward defence spending goals.
Italy and Spain, two of Europe's larger economies, spend barely 1 percent a year on defence as they seek to curtail budget deficits that swelled during the boom that preceded the 2008/2009 global financial crisis.
Belgium, the Netherlands and Denmark, three other wealthy European nations are among the smallest spenders on defence, as their budgets are locked in to multi-year cycles and centre-left parties do not support higher defence spending, preferring to prioritise welfare and education.
But the Baltic states of Latvia and Lithuania, who fear a repeat of Crimea, are on course to meet the 2 percent goal, while Romania is also heading towards that level.
After Mattis' address, Britain's Defence Secretary Michael Fallon said Mattis had explained that "the impatience of the American taxpayer is a reality".
Fallon said he called on all allies at the meeting to at least increase defence spending annually, even if they cannot meet the 2 percent target.
"We are asking them to commit to an annual increase, which would at least demonstrate good faith," he said. "Nineteen members of the alliance don't reach 1.5 percent and five of those 19 don't reach 1 percent."