Europe has hit back after US President Donald Trump announced plans to impose hefty tariffs on steel and aluminium.
France, Germany and the European Union have all raised objections to the proposals, while China urged restraint.
Trump later said trade wars were "good and easy to win".
The EU said the US had proposed imposing a 25% import duty on steel from Europe and elsewhere, and 10% on aluminium.
European Commission President Jean-Claude Juncker said the move would threaten thousands of European jobs and that the bloc will react to protect its own interests.
He 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 a solution, this move can only aggravate matters.
“The EU has been a close security ally of 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. The Commission will bring forward in the next few days a proposal for WTO-compatible countermeasures against the US to rebalance the situation."
Juncker’s comments raise the prospect of a trade war, something German government spokesman Steffen Seibert said was not in anyone’s interests.
France’s finance minister Bruno Le Maire said the tariffs were unacceptable.
Trump tried to justify the tariffs by saying they would safeguard American jobs in the face of cheaper foreign products.
"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," Trump tweeted on Friday.
The prospect of retaliation from Canada, China and Europe sent world stocks tumbling towards a 2.5 percent weekly loss as investors turned to traditional safe havens such as government bonds, gold and the Japanese yen.