Trump has repeatedly criticised NATO allies for failing to meet a 2014 commitment to spend two percent of GDP on defence by 2024, accusing them of leaving the US to shoulder an unfair burden for defending Europe.

Less than a fortnight before a NATO summit in Brussels, Trump has written individual letters to eight European allies to remind them of their pledge.

"I am not very impressed by this type of letter," Michel told reporters at an EU summit in Brussels.

"Belgium has halted the systematic fall in defence spending and takes part in a lot of military operations."

A diplomatic source told AFP the eight European countries to receive the letters, signed by Trump himself, were Belgium, Italy, Spain, Portugal, Luxembourg, Norway, the Netherlands and Germany.

Trump has written a similar letter to Canada, the text of which was made public last week, saying there was "growing frustration in the US that key Allies like Canada have not stepped up defense spending as promised".

Currently, the US accounts for nearly 72 percent of all defence spending in NATO and only three European countries hit the two percent GDP target -- Britain, Greece and Estonia.

Alliance officials are hopeful that four more will join the list by the July 11-12 summit -- most likely Poland, Romania, Latvia and Lithuania.