"Many countries in NATO, which we are expected to defend, are not only short of their current commitment of 2% (which is low), but are also delinquent for many years in payments that have not been made," he said, tweeting from the air en route to a key NATO summit in Brussels. "Will they reimburse the U.S.?"

In recent months Trump has repeatedly launched invective at America's partners in the Atlantic alliance, a cornerstone of European security since World War II, accusing them of freeloading on the US for defense.

The 29 allies are due to restate a commitment to try to spend two percent of GDP on defense by 2024, according to excerpts of their final summit declaration seen by AFP.

NATO chief Jens Stoltenberg said Tuesday that seven out of 27 European members were on course to spend two percent of GDP on defense in 2018: Britain, Greece, Latvia, Estonia, Poland, Lithuania and Romania.

The US is by far NATO's biggest spender in terms of GDP as well as raw expenditure, shelling out some 3.57 percent of GDP on defense in 2017, according to NATO figures.

According to alliance figures, the US accounted for just under 72 percent of defense spending in NATO in 2017 -- $686 billion of a total $957 billion spent by all 29 members that year.