EU trade chief Cecilia Malmstrom will put Europe's case to be permanently exempted from US tariffs for a final time on Monday, ahead of a pending US decision on whether to impose import duties on steel and aluminium coming from the bloc.

US President Donald Trump last month granted the European Union, and countries such as Canada and Mexico, a temporary exemption from the tariffs of 25 per cent on steel and 10 per cent on aluminium. Those exemptions expire on May 1.

If the EU is subject to tariffs on the €6.4 billion of the metals it exports to the United States, it has said it will set its own duties on €2.8 billion of US exports of products from make-up to motorbikes.

The European Commission, which oversees trade policy for the 28 EU members, has insisted that the United States grant it a permanent exemption without conditions.

European Trade Commissioner Malmstrom was due to talk by phone with US Commerce Secretary Wilbur Ross on Monday in a last effort to persuade Washington to do so. It was not clear whether she would make any particular offer.

"The only thing that I can tell you today is that we are patient but we are also prepared," Commission spokesman Margaritis Schinas told a news conference.

If the EU is subject to tariffs on the €6.4 billion of the metals it exports to the United States, it has said it will set its own duties on €2.8 billion of US exports of products from make-up to motorbikes.

Economists say the stand-off could tip towards a trade war if Trump responds with further tariffs, such as on EU cars.