The US has said it intends to negotiate three separate trade agreements with Japan, the UK and the EU.

"We will continue to expand US trade and investment by negotiating trade agreements with Japan, the EU and the United Kingdom," US trade representative Robert Lighthizer said.

"We are committed to concluding these negotiations with timely and substantive results for American workers, farmers, ranchers, and businesses.

The US plans to start negotiations with Japan, the world's third largest economy, "as soon as practicable, but no earlier than 90 days from the date of this notice," Mr Lighthizer wrote in a letter to Congress.

Negotiations with the United Kingdom will start "as soon as it is ready" after it leaves the European Union in March 2019, he said.