President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Friday pledged to respond to China’s latest round of tariffs “this afternoon,” further ratcheting up the trade war between Washington and Beijing.

In a string of tweets sent Friday morning, Trump also said he was ordering U.S. companies to “immediately start looking for an alternative to China,” proposing they begin making their products in the United States, though it was not immediately clear what authority he was attempting to invoke. Trump has previously pressured companies including Apple to begin producing their goods in the U.S.

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“The vast amounts of money made and stolen by China from the United States, year after year, for decades, will and must STOP. Our great American companies are hereby ordered to immediately start looking for an alternative to China, including bringing … your companies HOME and making your products in the USA,” Trump tweeted.

“I will be responding to China’s Tariffs this afternoon. This is a GREAT opportunity for the United States,” he tweeted.

....your companies HOME and making your products in the USA. I will be responding to China’s Tariffs this afternoon. This is a GREAT opportunity for the United States. Also, I am ordering all carriers, including Fed Ex, Amazon, UPS and the Post Office, to SEARCH FOR & REFUSE,.... — Donald J. Trump (@realDonaldTrump) August 23, 2019

Claude Barfield, an expert in international trade policy at the American Enterprise Institute (AEI), said in an interview that Trump doesn’t have any authorities to direct U.S. companies to move their businesses to the United States. He called the tweets a clear example of Trump “popping off” and predicted his advisers would look to do damage control, especially if his remarks negatively impact the stock market.

The messages came after China said earlier Friday that it would place new reciprocal tariffs on $75 billion in American auto parts and other goods.

Trump is facing mounting pressure over his trade war with China, which economists have blamed in part for contributing to the weakening of the global economy and causing uncertainty that has left the U.S. stock market volatile. An increasing number of economists have also predicted the U.S. could slide into a recession by the end of 2021.

U.S. stocks dropped sharply Friday after Trump declared he would respond to the new round of tariffs.

Trump and his advisers have insisted the economy is strong, pointing to low unemployment and other indicators, and sought to minimize concerns over the economy as speculation inflamed by the media.

Trump, who will soon depart Washington for Biarritz, France, for the annual summit of the Group of 7 (G-7) large economies, fired off several tweets Friday morning, insisting the economy is “strong and good.” Trump also took aim at Federal Reserve Chairman Jerome Powell, whom he regularly criticizes for keeping interest rates high, suggesting he was an "enemy" of the United States.

....My only question is, who is our bigger enemy, Jay Powell or Chairman Xi? — Donald J. Trump (@realDonaldTrump) August 23, 2019

Trump also said he would order Fed Ex, Amazon, UPS and the U.S. Post Office to “SEARCH FOR & REFUSE” deliveries of the synthetic opioid fentanyl from China and other countries. The Trump administration has taken aim at China as part of its effort to combat the opioid crisis, this week imposing sanctions on three Chinese nationals accused of trafficking synthetic opioids.

Updated at 2:37 p.m.