Trump said on Saturday he would impose a 30 per cent tariff on most imports from the EU and Mexico from August 1, adding to similar warnings for other countries and leaving them less than three weeks to hammer out framework deals that could lower the threatened tariff rate.
White House Economic Adviser Kevin Hassett said on Sunday that countries' trade deal offers so far have not satisfied Trump and "the tariffs are real" without improvements.
"The president thinks that deals need to be better," Hassett told US broadcaster ABC.
"And to basically put a line in the sand, he sent these letters out to folks, and we'll see how it works out."
Ursula von der Leyen, head of the EU's executive Commission which handles trade policy for the 27 member states, said the bloc would maintain its two-track approach: keep talking and prepare retaliatory measures.
"We have always been very clear that we prefer a negotiated solution. This remains the case, and we will use the time that we have now," von der Leyen told a press conference, adding that the bloc would extend its halt on countermeasures until August.
Von der Leyen's decision to resist immediate retaliatory measures points to the European Commission's desire to avoid a spiralling tit-for-tat escalation in the trade war while there remains a chance of negotiating an improved outcome.
German Chancellor Friedrich Merz on Sunday said he was "really committed" to finding a trade solution with the US, telling German public broadcaster ARD that he will work intensively on this with von der Leyen and French President Emmanuel Macron over the next two and a half weeks.
Asked about the effects of a 30 per cent US tariff on Germany, Merz said: "If that were to happen, we would have to postpone large parts of our economic policy efforts because it would interfere with everything and hit the German export industry to the core."
The latest salvo from Trump and the question of how to respond may test the unity of EU member states, with France appearing to take a tougher line than Germany, the bloc's industrial powerhouse whose economy leans heavily on exports.
Macron said the Commission needed more than ever to "assert the Union's determination to defend European interests resolutely," and that retaliation might need to include so-called anti-coercion instruments.
German Finance Minister Lars Klingbeil said on Sunday the EU should be ready to take firm action if talks failed.
"If a fair negotiated solution does not succeed, then we must take decisive countermeasures to protect jobs and companies in Europe," Klingbeil, also vice chancellor in the ruling coalition, told Sueddeutsche Zeitung newspaper.
