Last week, he told a crowd in Montana, "I'll tell NATO, you got to start paying your bills." The president has questioned the value of the US paying for the collective defense of Germany.

Any divisions between NATO allies during Trump's visit to Brussels could adversely affect the scheduled meeting between Russian President Vladimir Putin and Trump on Monday.

Bennett added: "The worst scenario from the American point of view is if there's a critical tone (from Trump) of the NATO alliance, and days later there's an uncritical tone of Putin and Russia. That just achieves Putin's agenda, not the United States' agenda.

"The best outcome is if the NATO allies agree to more commitments on defense spending, and Trump recognizes that－that would be progress."

Trump will meet British Prime Minister Theresa May, Queen Elizabeth II and prominent business leaders during his four-day trip.

The US Commander-in-Chief is set to arrive on Air Force One at London Stansted Airport on Thursday evening after attending the NATO summit in Brussels. From there, he will go to Blenheim Palace in Oxfordshire, the birthplace of Churchill, where May will host the president and his wife for a black-tie dinner with 100 guests and business leaders.

Trump will spend the night at the residence of the US ambassador in central London, and on Friday the president and May will attend a demonstration of the UK's cutting-edge military capabilities and integrated UK-US military training.

After that comes a working lunch at Chequers－the prime minister's official country residence－for what Downing Street says will be "substantive bilateral talks" covering a "range of foreign policy issues." These could include trade tariffs, Russia and North Korea.

The queen will host Trump at Windsor Castle later on Friday. He is expected to visit his golf course in Turnberry, Scotland, on Saturday before heading back to mainland Europe on Sunday ahead of the historic meeting with Putin in Helsinki on Monday.

Gaoyang Wange in London and Ying Wang in Washington contributed to this story.