The President's defence chief Jim 'Mad Dog' Mattis issued the warning during a tense meeting in Brussels earlier today.
He insisted the United States would pay its fair share of money into the transatlantic alliance's budget.
But he added: "If your nations do not want to see America moderate its commitment, each of your capitals needs to show support for our common defence."
Mr Mattis added: "No longer can the American taxpayer carry a disproportionate share of the defence of Western values.
"Americans cannot care more for your children's future security than you do."
President Trump has previously branded Nato "obsolete", sparking fears his administration could quit the alliance.
Donald Trump's defence chief issued the treat to Nato during talks in Brussels Being President Trump: Key Moments Fri, February 3, 2017
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Jim 'Mad Dog' Mattis with his UK counterpart Sir Michael Fallon
As President Trump has stated, he had strong support for NATO
But Mr Mattis reaffirmed the US' commitment to NATO while speaking to reporters in the Belgian capital.
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He told reporters: "The alliance remains a fundamental bedrock for the United States and for all the transatlantic community.
"As President Trump has stated, he had strong support for NATO."
Mattis at a press conference NATO Secretary General Jens Stoltenberg
And his British counterpart Sir Michael Fallon said: "There is no uncertainty about America's commitment to NATO.
"The president reassured our Prime Minister that he is 100% committed to NATO.
"I am looking forward to working with US defence secretary James Mattis on the steps needed to modernise NATO."
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