US president Donald Trump, who has repeatedly pledged to revive American manufacturing, said on Wednesday "big news" was coming that would be welcomed by US auto workers.

The tweet, seen by just-auto, reads: "There will be big news coming soon for our great American Autoworkers. After many decades of losing your jobs to other countries, you have waited long enough!"

The White House declined to comment to Reuters and the office of the US trade representative referred the news agency's queries to the White House.

Automakers scrambled after the tweet to get details. Some of them speculated that Trump could be referring to trade with the European Union, the news agency said.

In a meeting earlier this month with major automakers, Trump threatened to impose 20% or 25% tariffs on EU-built vehicles, according to three Reuters sources briefed on the talks.

Trump has repeatedly criticised the large trade imbalance on vehicles between the US and the EU. In March, he threatened in a tweet to "simply apply a tax on their cars which freely pour into the US".

The US currently imposes a 2.5% tariff on cars assembled in Europe and a 25% tariff on European-built vans and pickup trucks. Europe imposes a 10% tariff on US-built cars.

Reuters noted auto trade has been a big sticking point in talks between the US, Mexico and Canada as they try to update the North American Free Trade Agreement (NAFTA).

The US and Mexico have deadlocked over US demands for wage increases in the auto sector and for a boost in the North American content in cars. Last week, the top US trade official said the three countries were "nowhere close to a deal".

At the same time, Washington and South Korea are working to finalise an updated US-Korean free trade agreement.

Under an agreement-in-principle, South Korea has agreed to an extension of a 25% US tariff on pickup trucks for another 20 years until 2041. These were due to begin phasing out in 2019, causing concerns that South Korea would soon begin exporting pickup trucks to the US, Reuters said.

Trump hinted at the possibility of good news" on trade with South Korea during a meeting this week with South Korean president Moon Jae-in.

"We will have some pretty good news, I think, on trade," he told Reuters as the two leaders sat down.

UPDATE: Trump administration considering 25% tariff on imported vehicles