There has been swift reaction to Donald Trump's plan to impose steep new tariffs on steel and aluminium imports into the United States.

Dire warnings are being sounded of its potential effect on trade, and thousands of jobs worldwide, although the US president is standing by his plan.

Trump said on Thursday he would formally announce duties of 25 percent on steel and 10 percent on aluminium next week, although White House officials later said some details still needed to be ironed out.

He said he believes the tariffs will safeguard American jobs, but many economists say the impact of price increases for users of steel and aluminium, such as the car and oil industries, will destroy more jobs than curbs on imports create.

The announcement sent Asian stocks into free fall, with investors - and members of Trump's Republican party - warning of a global trade war.

But Trump argues the moves will rebuild US industry, and that other countries had treated them unfairly for decades.

When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win. Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy! — Donald J. Trump (@realDonaldTrump) March 2, 2018

"He was making good on a campaign promise to revive US manufacturing and address a trade imbalance with China, which saw him win big in Rust Belt states like Pennsylvania where he promised to bring back jobs," Al Jazeera's Kristen Saloomey, reporting from New York, said.

"But it's unclear whether the tariffs will apply to other trade partners like Canada from whom the US imports the most steel."

In fact, Canada has said it will retaliate against any restrictions on trade - and its views have been echoed by the European Union.

Global trade conflict

Germany has criticised strongly the US decision. Sigmar Gabriel, the foreign minister, said such punitive tariffs would mostly affect European industries, warning that it might also spark an international trade conflict.

"The EU has to respond in a determined manner to punitive tariffs by the US which would jeopardise thousands of jobs in Europe. There should be no doubt about that for those in Washington," Gabriel said in a written statement.

He said that a confrontation between the European Union and the US over trade affairs would be in the interests of neither the Europeans nor the Americans.

"When two people quarrel, a third rejoices. Therefore I hope that the president would reconsider his announced plans. We must do everything to prevent an international trade conflict," Gabriel said.

Separately, Bruno Le Maire, France's finance minister, said on Friday that Europe would retaliate with a firm joint response if the Trump administration goes ahead with "unacceptable" tariffs on imported steel and aluminium.

Le Maire said that "all options are on the table" including retaliation before the World Trade Organisation and countermeasures to limit European imports of US goods.

"The United States needs to know that if it goes ahead with these measures, they will meet with a strong, coordinated and united response from the European Union," Le Maire said at the ministry.

"These unilateral measures are not acceptable."

Referring to the Luxembourg-based European steel producer ArcelorMittal, he said they would have a major impact on the European economy and French companies like Vallourec and Arcelor.

Le Maire said Europe and the US needed to jointly tackle the problem of some countries heavily subsidising their producers, which makes it possible to dump their products abroad at prices below the cost of production.

"A trade war between Europe and the United States will only have losers," Le Maire said, adding that he would call his German and British counterparts on Friday to discuss the matter.

'Blatant intervention'

Alexander Winterstein, European Commission deputy chief spokesperson, said: "We strongly regret this step, which appears to represent a blatant intervention to protect US domestic industry and not to be based on any national security justification.

"Protectionism cannot be the answer to our common problem in the steel sector. Instead of providing the solution, this move can only aggravate matters. The EU has been a close security ally to the US for decades.

"We will not sit idly while our industry is hit with unfair measures that put thousands of European jobs at risk. The EU will react firmly and commensurately to defend our interests."

For his part, Hiroshige Seko, Japan's minister of economy, trade and industry, said: "We hope to find the opportunity to tell the United States that steel and aluminium imports from Japan, which is an allied nation, pose absolutely no threat to its national security."

Meanwhile, Hua Chunying, China's ministry of foreign affairs spokesperson, said: "The US has already taken at least 100 countermeasures against imported steel and aluminium products, providing excessive protection for domestic markets.

"If all countries followed the US example, this would result in a serious impact on the international trade order."