The new US defence secretary has told NATO members that Washington will "moderate its commitment" to the alliance if they do not increase their spending on defence, the BBC reports.
James Mattis's comments repeat President Donald Trump's demand that many countries raise their spending.
Earlier, he had hailed NATO as the "fundamental bedrock" of trans-Atlantic co-operation.
Mattis met NATO defence ministers for the first time on Wednesday.
"No longer can the American taxpayer carry a disproportionate share of the defence of Western values," he said at the Nato headquarters in Brussels, according to a text of his remarks.
"Americans cannot care more for your children's future security than you do."
Trump's comments, during his campaign, that the US might not defend allies who do not contribute their "fair share" to NATO, had worried many European nations, particularly those near Russia's border.