US president Donald Trump has threatened the European Union with taxes on its car exports to the US if the bloc fails to play fair on trade.

Trump repeated his line that the EU was treating the US unfairly on trade, shutting out US farmers, according to a Reuters report.

"That'll change also and I think we'll see that because on 25 July they're coming in to start negotiations with me. We'll see," he told a news conference at the end of a meeting of NATO leaders.

"And if they don't negotiate in good faith we'll do something having to do with all the millions of cars that are coming into our country and being taxed at a virtually zero level, at a very low level... I think it's been a very effective way of negotiating, but I'm not negotiating, I just want fairness for the United States," Reuters quoted Trump as saying.

The president arrived in the UK on Thursday (12 July) on a visit billed as a working trip rather than a state visit. He did not speak to waiting media on arrival at Stansted Airport and was whisked by helicopter to the new US embassy in London shortly afterwards.