U.S. and Polish army soldiers
© Kacper Pempel / Reuters
US Defense Secretary James Mattis delivered an ultimatum to NATO allies during a visit to Europe - increase their military spending, or see America "moderate its commitment" to the 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.

"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 defense."

"No longer can the American taxpayer carry a disproportionate share of the defense of western values," Mattis said. "Americans cannot care more for your children's security than you do. Disregard for military readiness demonstrates a lack of respect for ourselves, for the alliance, and for the freedoms we inherited, which are now clearly threatened."

The remarks were delivered during a closed-doors meeting with NATO defense ministers in Brussels, and provided to reporters traveling with the new Pentagon chief.

Earlier on Tuesday, Mattis referred to NATO as "a fundamental bedrock for the United States and for all the transatlantic community" and reiterated President Donald Trump's "strong support" for the alliance.

During the 2016 presidential campaign, Trump has criticized NATO as "obsolete" and singled out the lack of military spending as proof that the US was "paying disproportionately" to maintain the alliance.

"I think NATO as a concept is good, but it is not as good as it was when it first evolved," Trump told the Washington Post in March 2016. "NATO is costing us a fortune and yes, we're protecting Europe - but we're spending a lot of money."

In an interview to the New York Times in July 2016, Trump said there were "many countries that have not fulfilled their obligations to us," referring to NATO allies whose military spending fell below the 2 percent of GDP.

Currently only four European NATO members - UK, Estonia, Poland and Greece - spend at least 2 percent of their gross domestic product on the military. Major alliance members such as France, Germany, Italy, Turkey and Canada would all need to boost their military spending to meet the benchmark, set by the Obama administration.