US personnel were among 31 nations to take part in Exercise Trident Juncture recently (Picture: JFC Brunssum Imagery via US Department of Defense).
US President Donald Trump says that other nations should "pay the United States for its great military protection, or protect themselves", after returning from meetings in France.
Mr Trump posted a series of tweets and stated his view that the US "must be treated fairly" on defence and trade.
Posting to Twitter he said: "We pay for large portions of other countries (sic) military protection, hundreds of billions of dollars.
"It is, and always has been, ridiculously unfair to the United States.
"It is time that these very rich countries either pay the United States for its great military protection, or protect themselves."
In July, the US President backed down over threats to pull the US out of NATO after other leaders agreed to increase defence spending "like they never have before" and stated his belief that "4% [of GDP] is the right number".
NATO's spending target for its members is two per cent of GDP, and in response to Mr Trump's assertion that four per cent was "the right number", a Downing Street spokesperson said the UK was "already paying above the 2% target".
It follows a tweet sent by Mr Trump last week which said suggestions by French President Emmanuel Macron over a European military were "very insulting".
In the same post, he went on to say: "Perhaps Europe should first pay its fair share of NATO, which the U.S. subsidizes greatly!"
American troops were among 31 nations taking part in NATO's recent Exercise Trident Juncture, which was the alliance's biggest exercise in more than a decade.
Earlier this year, MPs warned the government that spending on the armed forces should increase to three per cent of GDP, in order for the UK to maintain its defence relationship with the United States.