WASHINGTON — President Donald Trump declared Friday that he had “hereby ordered” American companies “to immediately start looking” for alternatives to operating in China, hours after Beijing announced new trade sanctions on U.S. products.

But as markets in the U.S., Asia and Europe tumbled, the White House offered no further details or explanation of Trump’s intentions.

Instead, the president tweeted that he would be “responding to China’s Tariffs this afternoon.”

“This is a GREAT opportunity for the United States” he wrote.

The rising tensions between the world’s two biggest economies unnerved investors already on edge about the deteriorating world economy. The Dow Jones industrial average dropped 623 points Friday as companies and business groups large and small joined in a chorus of opposition.

“There doesn’t seem to be a floor under the spiraling down of the relationship, and world markets are reacting negatively to a business climate beset by uncertainty and risk,” said Douglas Barry, spokesman for the U.S.-China Business Council.