Mattis said that if the other Nato countries don't live up to their spending commitments, the United States will be forced to “moderate its commitment” to the nearly 70-year-old alliance.
“I owe it to you all to give you clarity on the political reality in the United States, and to state the fair demand from my country's people in concrete terms,” Mattis said, according to numerous US media reports.
“America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to the alliance, each of your capitals needs to show its support for our common defence," he continued.
Nato has a defence spending target of two percent of a country's gross domestic product (GDP), something only five countries meet.
Norway currently uses 49 billion kroner on defence, amounting to 1.56 percent of its GDP. Hitting the two percent target would require spending an additional 13 billion kroner but as the Norwegian GDP grows, that price tag will only get larger with every passing year.
Military budgets were given a significant boost in the Norwegian Armed Forces long-term plan presented last year. The plan called for spending to increase to 53 billion kroner in 2020, but that is barely enough to keep pace with economic growth. The GDP ratio in 2020 is projected to be at 1.57 percent, just marginally higher than today.
State Secretary Øystein Bø from the Ministry of Defence, however, said that Norway is on the right path.
“But at the same time it will be difficult to bring the budget up to two percent so quickly. The opportunities will also depend on developments in the Norwegian economy in the period,” he said.
The two percent target was adopted at the Nato summit in Wales in 2014. Formally, it was formulated as a pledge to "move toward two percent" within a decade.
Norway has not announced any plan showing its anticipated defence spending throughout the decade. The Ministry of Defence has kept its calculations hidden in a document classified as ”strictly confidential".
News agency NTB, however, has made its own calculations based on the what it knows about the Ministry of Defence's GDP growth expectations. By those estimates, Norway would have to increase military spending by roughly 48 percent in 2024 to reach the two percent target that year.
US demands that its Nato allies increase their defence spending are nothing new but they have been given increased significance under President Donald Trump, who has cast doubts on Nato itself by branding the Western military alliance “obsolete”.
Speaking in Brussels on Wednesday, Mattis reaffirmed Washington's commitment to Nato but the former Marine was stern in his rebuke of member states for not paying their fair share.
"Americans cannot care more for your children's future security than you do," he said.
The US currently spends 3.6 percent of its GDP on the military, amounting to $664 billion in 2016.