US Defence Secretary James Mattis bluntly warned NATO allies today that Donald Trump's administration would "moderate its commitment" to the transatlantic alliance unless members met their spending pledges.
Mattis's no-nonsense message to his counterparts in Brussels follows years of calls by Washington for allies to spend at least two percent of their GDP on defence, a goal that only a handful meet despite agreeing on it at a summit in 2014.
"Americans cannot care more about your children's future security than you do," retired Marine general Mattis said in prepared remarks to defence ministers at NATO headquarters.
"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 called for "milestone dates" this year that would track NATO member contributions.
NATO Secretary General Jens Stoltenberg denied that Mattis's words amounted to a threat, saying it was a "firm message" and that his colleagues agreed that they would do more to contribute.
"Secretary Mattis conveyed a very firm message to all Allies. That message is about the importance of fair burden-sharing. It reflects a political reality in the United States," Stoltenberg told a press conference.
Currently, only the United States, Britain, Estonia, Greece and Poland have hit or surpassed the two percent figure.
The call for more funding comes as NATO mounts its biggest buildup since the Cold War, strengthening its eastern flank following Russia's annexation of Crimea in 2014.
British Defence Secretary Michael Fallon told reporters that he had called on laggards to at least increase spending annually to "demonstrate good faith."
NATO said its member countries spent a total of USD 892 billion in 2015, with the United States accounting for USD 641 billion. Russian military spending in 2015 rose to 66 billion dollars, the SIPRI institute in Stockholm said.
Despite lambasting some NATO partners' spending, Mattis hailed NATO as the "fundamental bedrock" of transatlantic security as he sought to reassure allies about Trump's commitment to the alliance.
Since his inauguration, Trump has taken a more orthodox stance on NATO and reaffirmed long-standing US commitment to the alliance that Mattis has previously served.
Trump, who is set to meet NATO leaders in Brussels in May, had previously said the alliance was "obsolete.