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.
James Mattis’s comments repeat President Donald Trump’s demand that members raise their spending to meet a target of 2% of their GDP.
Only five of the 27 countries do so.
Earlier, Mr Mattis had hailed Nato as the “fundamental bedrock” of trans-Atlantic co-operation.
According to a text of his remarks, Mr Mattis said at the Nato headquarters in Brussels: “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,” he added, as he met defence ministers for the first time.
Commentators said this was some of the strongest criticism in years to allies who have not hit their spending goals. Mr Mattis said members must show progress this year and adopt a plan to increase their contributions, even if slowly.
Only the US, UK, Estonia, Greece and Poland currently meet the target, but others are on course to reach that level.
“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,” Mr Mattis said.
Nato Secretary General Jens Stoltenberg said Nato members were already increasing their contributions.
Mr 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.
He was critical of the Western military alliance, describing it as “obsolete”.
But Mr Mattis showed strong support for the alliance, hailing its ability to respond to security challenges.
“The alliance remains a fundamental bedrock for the United States and the trans-Atlantic community, bonded as we are together,” he said.