Trump reignites tensions with EU tariff threats

Trade expert Aslak Berg from the Centre for European Reform told the BBC that he thought Trump's post was intended to increase leverage ahead of the negotiations.

"But the fact of the matter is the EU is not going to budge. They are going to stay calm, carry on and it will be a very difficult discussion," he said.

He added that Trump's re-escalation of tariffs set a bad precedent for other deals the US might be working on.

"For a while there was this perception ... that Trump was backing off, that we would have a bit of quiet and stability. This shows that is not at all the case," he said.