President Trump said Tuesday that he is serious about imposing tariffs on European automobiles if he can’t strike a trade agreement with the European Union.

“They know that I’m going to put tariffs on them if they don’t make a deal that’s a fair deal,” Trump said in an interview with The Wall Street Journal on the sidelines of the World Economic Forum.

Mr. Trump declined to say what deadline he is imposing on the negotiations before he would move ahead with the auto tariffs. “They know what the deadline is,” the president said, adding that he would reveal it publicly soon.

The remarks came on Trump’s first day at the annual summit of world leaders and business executives. He spoke to the Journal shortly before participating in a bilateral meeting with European Commission President Ursula von der Leyen. He didn’t mention auto tariffs in brief remarks to reporters during the meeting.

The Trump administration let a previous deadline of Nov. 13 for the imposition of auto tariffs lapse. Former European Trade Commissioner Cecilia Malmstrom said last year the EU considered that lapsed deadline as an indication Washington wouldn’t enact the duties on auto imports from Europe. She said the EU would strike back with duties on American exports.

The president also confirmed that he has come to an agreement with French President Emmanuel Macron to postpone until the end of 2020 the tax that France levied on big technology companies last year. Trump said he threatened to impose a 100% tax on French wine if the digital tax wasn’t paused.

“It’s not that I’m in love with them,” Trump said of U.S. tech companies, “but if anybody is going to tax these companies, it’s going to be U.S. that taxes these companies.”

The president also said his administration is developing a middle-class tax cut that would go into effect if he keeps the presidency and Republicans control Congress after the 2020 elections. He declined to offer any details about the measure.

“We’re talking a fairly substantial…middle-class tax cut that’ll be subject to taking back the House and obviously keeping the Senate and keeping the White House,” he said. The president said the administration is crafting it now and it will be unveiled in 90 days.

Trump confirmed that he is planning to add additional nations to an updated version of his travel ban that the administration is expected to release later this month. He wouldn’t reveal which countries would be added. The Supreme Court in 2018 affirmed Trump’s power to block citizens of several Muslim-majority nations from entering the U.S., finding the authority was justified on independent national-security grounds including an administration study that found weaknesses in how some countries vetted their travelers.

The president also said he didn’t know Greta Thunberg, the teenage Swedish climate activist whom Trump criticized in a December tweet for what he described as an “anger management problem.” Thunberg is speaking at the Davos summit as well.

“I don’t really know anything about her,” Trump said before adding that she’s “very angry.”

The president, in a roughly 30-minute speech earlier Tuesday at the summit, touted the U.S. economy. He also said the passage of a new trade agreement with Canada and Mexico, and last week’s signing of an early-stage deal with China represent a model of future trade negotiations.

Asked to explain his top-line message to the summit attendees, Trump said: “How well the U.S. is doing.”

“The potential is enormous. There’s tremendous growth potential,” he said, adding that the effects of the trade agreements haven’t kicked in yet.

An expanded version of this story appears on WSJ.com.