President Donald Trump on Friday said he was ordering U.S. companies to "immediately start looking for an alternative to China, including bringing your companies HOME and making your products in the USA."

Trump also said he was ordering all U.S. postal carriers, including FedEx, Amazon, UPS and United States Post Office, "to SEARCH FOR & REFUSE all deliveries of Fentanyl from China (or anywhere else!)."

And Trump said he will respond this afternoon to China's newest round of tariffs on U.S. goods.

The White House did not immediately respond when asked if the announcement, delivered in a four-part Twitter thread Friday morning, constituted an official order from the president.

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It was not immediately clear how, or under what authority, the president could implement these declared orders, or whether he had already done so.

Stocks sank to session lows shortly after Trump's tweets. The Dow Jones Industrial Average fell more than 435 points, or 1.6%, while the S&P 500 slid 1.7% and the Nasdaq Composite dove 2%.

In a statement, UPS said that it "follows all applicable laws and administrative orders of the governments in the countries where we do business. We work closely with regulatory authorities to monitor for prohibited substances."

FedEx also responded: "FedEx already has extensive security measures in place to prevent the use of our networks for illegal purposes. We follow the laws and regulations everywhere we do business and have a long history of close cooperation with authorities."

Amazon and the Postal Service were not immediately available for comment.

Trump's tweets followed another missive against Federal Reserve Chairman Jay Powell, who had just pledged to "act as appropriate" to sustain the U.S. economy amid the "deteriorating" global economic outlook.

In an apparent response, Trump tweeted: "Who is our bigger enemy," Powell or Chinese President Xi Jinping?