British Prime Minister Theresa May told US President Donald Trump that US tariffs imposed on European Union steel and aluminum imports were "unjustified and deeply disappointing" during a phone call on Monday.

In a statement, May's office said: "The US, UK and EU are close national security allies who should recognize the value of open and fair trade across the world."

May underlined the need to safeguard jobs and they agreed to discuss the issue further at the G7 summit in Canada later this week.

Trump last week slapped tariffs on imported steel and aluminum, a move that free trade supporters said could spark a trade war.

At the weekend China warned that all trade talks between Beijing and Washington would be void if the US sets up trade sanctions.

The EU threatened tariffs on Harley-Davidson motorcycles and bourbon, measures aimed at the political bases of US Republican legislators.

May is the second EU leader in four days to slam the administration's protectionist trade policies.

The US tariffs of 10 percent on aluminum and 25 percent on steel from the EU, Canada and Mexico have been widely condemned. The EU, Mexico and Canada have all set out plans for retaliatory tariffs on US goods.

French President Emmanuel Macron has called Trump to tell him the tariffs were "illegal", while Canadian Prime Minister Justin Trudeau has said the US move is "totally unacceptable".

A phone call on Thursday between Trump and Macron went badly when the French president confronted the commander-in-chief over trade and immigration, CNN reported.

Britain's International Trade Secretary Liam Fox warned Parliament on Monday that growing protectionism could "ramp up into a global trading disaster" and said Britain and the EU's response must be measured and proportionate.

According to a White House statement, Trump also called for a new and comprehensive deal toward Iran.

Trump announced the US' exit from the 2015 Iran nuclear deal in May, despite European nations' protests.

It said Trump also "underscored the need to rebalance trade with Europe" and expressed hope for a Brexit deal that does not increase tensions on the Northern Ireland border.

On Saturday, Trump tweeted that the US had been "ripped off by other countries for years on trade".

He said steel tariffs will protect US steel-makers, which he said are vital to national security. Trump has also complained about barriers US companies face in Europe and elsewhere. "Time to get smart!" he added.

The US State Department on April 26 confirmed that Trump will pay a working visit to Britain on July 13. The visit would start after Trump attended the NATO summit slated for July 11-12 in Brussels.