Trump fires warning shot at allies before leaving for NATO meeting: 'US must pay less -- very unfair!'
President Donald Trump. (AFP / Mandel Ngan)

President Donald Trump railed against NATO allies before boarding Air Force One for the annual meeting in Europe.

The president has ramped up his complaints about the decades-old alliance in recent weeks, and he fired off two tweets early Tuesday before departing for the meeting with allied leaders.

"Getting ready to leave for Europe. First meeting - NATO," the president tweeted. "The U.S. is spending many times more than any other country in order to protect them. Not fair to the U.S. taxpayer. On top of that we lose $151 Billion on Trade with the European Union. Charge us big Tariffs (& Barriers)!"

Trump will also visit Great Britain and then Helsinki, Finland, for a private, one-on-one-meeting with Russian president Vladimir Putin, whose country's most important strategic goal is the weakening of NATO.

"NATO countries must pay MORE, the United States must pay LESS. Very Unfair!" Trump tweeted.

The president also retweeted a post welcoming first lady Melania Trump to Britain by Trump supporter Shenna Fox, a Scotland-born pianist who was convicted of theft as part of a gambit to perform at Carnegie Hall.

Fox went into debt to cover a $21,000 fee associated with the performance, and she forged $3,360 worth of Choral Union checks in an attempt to pay off her debt.

She was sentenced to perform 180 hours of community service, and Fox and her husband were thrown out of the Choral Union.

Trump has previously retweeted a conspiracy theory floated by Fox about a man who allegedly threatened former porn actress Stormy Daniels over her claims of a sexual relationship with the president.