Leaders around the world expressed their concerns after US President Donald Trump imposed 25 percent tariffs on foreign steel and aluminum.

He signed proclamations on the new steep rates in the Oval Office on Monday evening. They will take effect on March 12.

European Union officials expressed their frustration. EU Trade Commissioner Maros Sefcovic said Tuesday that they will respond in a "firm and proportionate way by countermeasures."

Sefcovic also noted that tariffs are not only harmful for the trading partners directly involved, but also risk having disruptive effects for many others, as well as the global trading system. "Put simply, it is a lose-lose scenario," he said.

South Korean acting President Choi Sang-mok said he will discuss response measures with the EU and Japan. He said his government will prepare necessary assistance for affected companies. He also announced that a group of CEOs from top South Korean conglomerates plan to visit the US.

Trump also spoke with Australian Prime Minister Anthony Albanese by phone prior to the announcement. He suggested he may go easier on the country, saying: "We actually have a surplus. It's one of the only countries which we do. And I told him that that's something that we will give great consideration to."

The president has repeatedly claimed that tariffs help protect American steelmaking and jobs.