"They can all afford it, but they should be at five percent not two percent," the incoming US president told reporters.

Trump has long been skeptical of NATO, the cornerstone of security in Europe since World War II, and last month reiterated a familiar threat to leave the alliance if its members did not step up spending.

"If they're paying their bills, and if I think they're treating us fairly, the answer is absolutely I'd stay with NATO," the incoming US president said in an interview.

The transatlantic alliance's 32 countries in 2023 set a minimum level for defense spending of two percent of gross domestic product, and Russia's war in Ukraine has jolted NATO to strengthen its eastern flank and ramp up spending.

Trump is not the only top official to call for an increase -- NATO chief Mark Rutte likewise said last month that "we are going to need a lot more than two percent."

In his first major speech since taking over as NATO secretary-general, Rutte also warned that European nations were not prepared for the threat of future war with Russia, calling on them to "turbocharge" their defense spending.