US President Donald Trump has backed boycotting American motorcycle manufacturer Harley-Davidson, the latest salvo in a dispute between the company and Trump over tariffs on steel.

The American motorcycle manufacturer announced a plan earlier this year to move production of motorcycles for the European Union from the US to its overseas facilities to avoid the tariffs imposed by the trading bloc in retaliation for Trump's duties on steel and aluminium imports.

In response, Trump has criticised Harley-Davidson, calling for higher, targeted taxes and threatening to lure foreign producers to the US to increase competition.

"Many @harleydavidson owners plan to boycott the company if manufacturing moves overseas. Great! 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 said in a Twitter post on Sunday.

Harley-Davidson has repeatedly declined to comment on Trump's remarks over the course of the dispute. The company could not be immediately reached for comment on Sunday.

Harley has forecast that the EU tariffs would cost the company about $US30 million to $US45 million for the remainder of 2018 and $US90 million to $US100 million on a full-year basis.

Trump met on Saturday with a group of bikers who support him, posing for pictures with about 180 bikers at his golf resort in Bedminster, New Jersey, where he is on vacation.

Motorcycle companies based outside the US include Japan's Honda Motor Co and Yamaha Corp, Europe's BMW and Ducati as well as India's Hero MotoCorp, Bajaj Auto, among others.