European Commission chief Jean-Claude Juncker has vowed to fight back against US President Donald Trump's threat of a 25% tariff on steel and 10% on aluminium imports.

"So now we will also impose import tariffs. This is basically a stupid process, the fact that we have to do this. But we have to do it. We will now impose tariffs on motorcycles, Harley Davidson, on blue jeans, Levis, on Bourbon. We can also do stupid. We also have to be this stupid," he said in Hamburg on Friday evening.

While Trump may be comfortable with the idea of a trade war, it wasn't just across the Atlantic where the leader's plans ruffled feathers.

"We are impressing upon the American administration the unacceptable nature of these proposals that are going to hurt them every bit as much as they will hurt us," said Canadian Prime Minister Justin Trudeau.

The warnings from leaders around the world mirrored those of the International Monetary Fund, which said Trump’s plan would cause damage both internationally and within America itself.

Trump however remains defiant, insisting that trade wars are good and easy to win.