Trump has declared that the U.S. will impose steep tariffs on steel and aluminum imports, pushing back against allegedly unfair trade deals with China and other countries. With tensions rising over international trade, stocks closed sharply lower on Wall Street. China on Friday expressed "grave concern."

Trump said firm action was crucial to protect U.S. industry from unfair competition and to bolster national security. His announcement came only after an intense internal White House debate. It brought harsh criticism from some establishment Republicans as well.

Juncker said the EU would take their own action if Trump followed through.

While not immediately offering a specific response on what it would do, the Chinese Commerce Ministry statement said: "The Chinese side expresses grave concern." The ministry said they've appealed to Washington to settle disputes through negotiation