The European Union says United States President Donald Trump's plan to double import taxes on steel and aluminum products "adds further uncertainty to the global economy and increases costs for consumers and businesses on both sides of the Atlantic" and "undermines" efforts to bring an end to the wider tariffs stand-off.

"We strongly regret the announced increase of US tariffs on steel imports from 25 percent to 50 percent," the European Commission, which is the executive arm of the 27-member bloc, said on Saturday.

Trump's latest plan to raise duties on steel imports was announced at a rally in the steel-making US city of Pittsburgh on Friday.

At the gathering, he told local steelworkers they would soon be receiving "a well-deserved $5,000 bonus", as part of a program to decrease reliance on external suppliers, including the EU and China.

In a statement sent to the BBC, the European Commission said the latest announcement "undermines ongoing efforts to reach a negotiated solution", and warned that the EU was prepared to impose countermeasures.

According to the European economic think tank Bruegel, before the current outbreak of trade hostilities, "the average US tariff rate on imports from the EU was 1.47 percent".

When Trump unveiled his global tariff program in early April, imports from the EU became subject to 20 percent tariffs, which was later reduced to 10 percent to allow time for negotiations.

A separate 25 percent tariff had already been imposed on almost all steel imports to the US in March, before these wider duties were announced.

A week ago, Trump said that following what he called "a very nice call" with European Commission President Ursula von der Leyen, the negotiations deadline was being pushed back to July 9.

Von der Leyen adopted a similarly positive outlook and said the EU was "ready to advance talks swiftly and decisively", but stressed that time was needed for the best possible outcome.

The Euronews website quotes figures from the US Department of Commerce showing that since Trump began his second term of office as president in January, steel prices have risen by 16 percent. Steel costs $984 per metric ton in the US, as opposed to $690 in Europe.