Donald Trump stands with members of Bikers for Trump as they say the Pledge of Allegiance.

US President Donald Trump, using Twitter again to criticise a company he once hailed as a "true American icon", gave his backing to Harley-Davidson motorcycle owners who may boycott the manufacturer if it moves some production out of the US.

"Great," the US president said. "Most other companies are coming in our direction, including Harley competitors. A really bad move! US will soon have a level playing field, or better."

Trump has lashed out more than a half-dozen times against the Milwaukee-based maker of the iconic motorcycles after praising the company early in his presidency.

The relationship flipped after Harley announced plans earlier this summer to shift production of bikes destined for Europe to overseas facilities.

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The company acted after Trump's administration imposed steel and aluminum tariffs that promoted retaliatory measures from the European Union that Harley-Davidson said would increase its costs by US$2200 for each bike shipped there.

Harley doesn't sell motorcycles in the US that are built overseas, and the company has said that won't change.

CAROLYN KASTER/AP Trump has lashed out more than a half-dozen times against Harley-Davidson.

A Harley-Davidson spokesman didn't immediately respond to a voice mail and email seeking comment.

Trump's pronouncement came a day after he welcomed biker supporters to his golf course in Bedminster, New Jersey, and a New York Times article cited some Harley-Davidson owners criticising the company at a rally in Sturgis, South Dakota in the US.