China and the EU vowed on Wednesday to strike back and defend their economic interests against sweeping new US steel and aluminium tariffs, moving Washington closer to an all-out trade war with two major partners.

The levies took effect just after midnight on Wednesday "with no exceptions or exemptions", as promised by the White House – despite countries' efforts to avert them.

The European Commission said it would impose "a series of countermeasures" from April 1 in response to the "unjustified trade restrictions" from the United States.

"We deeply regret this measure," European Commission chief Ursula von der Leyen said in a statement, adding that "the countermeasures we take today are strong but proportionate".

"As the US are applying tariffs worth US$28 billion, we are responding with countermeasures worth" the equivalent in euros, she said.

And China, the world's leading steel manufacturer – though not a major exporter of the product to the United States – vowed "all necessary measures" in response.

"There are no winners in trade wars," foreign ministry spokeswoman Mao Ning said.

Washington's tariffs would "seriously damage the rules-based multilateral trading system", she warned.

Trump has imposed steep tariffs on major US trading partners Canada, Mexico and China since returning to office, allowing only a partial rollback for his country's neighbours while vowing fresh levies from April 2. (AFP)