Mattis said only five NATO members meet the target of spending 2 percent of their gross domestic product -- Britain, Estonia, Poland, Greece and the US -- while other nations lag considerably despite "benefiting from the best defense in the world."

"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 need to show support for our common defense," Mattis said.

The forceful demand contrasts sharply with what Mattis said while visiting South Korea and Japan early this month, his first overseas trip since taking office. During the trip, Mattis spoke only about how firmly the US is committed to the alliances and made no mention of burden-sharing.

That's in line with what President Donald Trump has said since the election.

During the campaign, Trump accused allies, including NATO, South Korea and Japan, of free-riding on American security commitments. Since his election, Trump kept up the burden-sharing demand for NATO nations but made no mention of such a demand for the Asian allies.

Experts say that NATO and South Korea are different in many aspects, including the fact that dealing with North Korean threats takes a higher priority than burden sharing in the alliance with Korea, though that does not mean Trump won't push Seoul to pay more. (Yonhap)