Staying true to President Donald Trump's rhetoric on the North Atlantic Treaty Organization, Defense Secretary and top Pentagon official Jim Mattis told member countries assembled in Brussels Wednesday they must increase defense spending or the US will "moderate its commitment", the Associated Press reported.
Mr Mattis told members to "show support for our common defence" if they "do not want to see America moderate its commitment to this alliance".
NATO's defense spending has increased by 3.8%, or $10 billion, among European allies and Canada, NATO Secretary General Jens Stoltenberg said at a press conference ahead of a meeting of the alliance's defense ministers.
Mattis' cryptic ultimatum prompted fears that the USA might no longer stand by allies if they came under attack, signaling a possible weakening of NATO's Article 5 policy that an attack on one member requires a response from all.
Now only four European NATO members -UK, Estonia, Poland and Greece - spend at least 2 percent of their gross domestic product on the military.
Just five of NATO's 28 member countries meet the requirement for spending 2% of their GDP on defense.
"And it's absolutely appropriate, as the European minister of defense said last week, it's a fair demand that all who benefit from the best defense in the world carry their proportionate share of the necessary cost to defend freedom", he added.
According to prepared remarks provided to journalists, Mattis said: "America can not care more for your children's future security than you do".
"I depart here confident that we have an appreciation of the burden sharing that we must all sustain for deterrence, peace and prosperity".
Jen Stoltenberg, speaking ahead of his meeting with USA defence chief James Mattis, promised to fix the alliance's funding "challenge" after a defence expert championed a "European pillar" of defence, worryingly echoing calls for an EU army.
Mattis met privately with defense ministers from other North Atlantic Treaty Organisation countries in Brussels.
And it said there had been "real and important" increases in Chinese military activity in the air, at sea, and in missile forces.
"The point about Russian Federation is they have to live by global laws just like you expect all mature nations on this planet to do", he said.
"NATO provides strong political support for Ukraine and strong practical support for Ukraine".
The meeting will also discuss Russian Federation as well as the threat from the Islamic State group.
Trump's predecessor Barack Obama slapped sanctions on Russia's FSB domestic agency and the GRU military intelligence over accusations they were involved in cyberattacks against the US.
"This is significant, but it is not enough", he said.
Mattis, at the outset of the meeting on the Islamic State effort, said that the mission is "not something that will over with quickly, but we certainly need to accelerate this fight". Many are already talking about increasing commitments, he added.