President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Tuesday suggested countries that haven't made their full contributions to fund NATO should reimburse the U.S., further stoking his feud with members of the alliance as he travels to Brussels to meet with NATO leaders.

"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. Will they reimburse the U.S.?" Trump tweeted.

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. Will they reimburse the U.S.? — Donald J. Trump (@realDonaldTrump) July 10, 2018

The president sent the tweet as he traveled on Air Force One to Belgium, where he will take part in the annual NATO summit with other world leaders that is scheduled for Wednesday and Thursday.

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Trump has long been critical of what he views as the U.S. taking on a disproportionate burden in funding NATO, but he has ratcheted up those complaints in the days leading up to the summit.

NATO members agreed in 2014 to move toward spending at least 2 percent of their respective gross domestic product on defense by 2024.

Trump reportedly wrote to German Chancellor Angela Merkel and other NATO allies last month to chastise them for failing to live up to their spending obligations. He further warned that the U.S. could alter its military deployments if nothing changes.

On Tuesday morning, just before he was set to depart for Brussels, Trump tweeted that the funding burden was unfair to American taxpayers, adding that other countries should pay more and the U.S. should pay less for NATO.

Trump went on to tell reporters on the White House lawn that he believes he'll be able to "work something out" with other member countries.

"NATO has not treated us fairly but I think we'll work something out. We pay far too much and they pay far too little," Trump said. "But we will work it out and all countries will be happy."