WASHINGTON – Reacting to a new round of Chinese tariffs imposed on U.S. goods, President Donald Trump said Friday he "hereby ordered" U.S. companies to "start looking for an alternative to China" – and he pledged to respond formally within hours.

In a series of tweets posted before he flies to France for the G-7 meeting of world leaders – where the global economy and trade issues will take center stage – Trump blasted China's latest action and vowed to announce retaliatory moves soon.

Markets plunged Friday as it became clear the trade war between Washington and Beijing was escalating. The Dow Jones Industrial Average was off by more than 500 points in midday trading, following a series of dramatic declines last week.

Trump said he would respond to the latest tariffs "this afternoon."

"Our great American companies are hereby ordered to immediately start looking for an alternative to China, including bringing our companies HOME and making your products in the USA," Trump wrote.

"We don’t need China and, frankly, would be far better off without them," he added.

White House officials did not respond to questions about what actions, specifically, Trump was referring to as pundits opined on social media that the president cannot easily order companies to pursue other markets. His posts came hours after Chinese officials announced in state media they will impose tariffs ranging from 5% to 10% on an additional $75 billion in U.S. goods, further increasing trade tensions.

Major business groups, including the U.S. Chamber of Commerce, rejected the notion of demanding businesses look to other markets.

"While we share the president’s frustration, we believe that continued, constructive engagement is the right way forward," Myron Brilliant, executive vice president and head of international affairs for the Chamber said in a statement. "Time is of the essence. We do not want to see a further deterioration of US-China relations."

Trump, who ran for office in 2016 in part on a promise to rework trade agreements he often described as "stupid," has argued that the dizzying series of tariffs imposed by both countries have hurt China far more than the United States. He repeated assertions this week that the tariffs have not had a major impact on U.S. consumers, arguing that Chinese companies are offsetting the cost by lowering prices.

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Trump's top trade advisers, who were seen at the White House before the president's tweets, scrambled to assure investors that negotiations with Chinawere ongoing. But Trump himself has repeatedly hinted that China might prefer to wait until after the 2020 election before locking down an agreement to end the trade dispute.

"In the two and a half years that the president’s been in office, he has absolutely galvanized the American people and Capitol Hill behind his China policy, and we know why. China steals our stuff," White House director of trade policy Peter Navarro told Fox Business on Friday. "They flood our communities with fentanyl and everything in between."

Trump also wrote that he would order U.S. shipping companies such as FedEx and UPS to search for and block shipments of fentanyl. Those companies are already required to provide additional information to U.S. border officials about packages in an effort to block the illicit drug, and Trump signed a measure last year to apply those same requirements to the U.S. Postal Service.

White House officials did not respond to questions about the fentanyl order.

Trump joked later that the markets were down not because of his tweets, or the ongoing trade dispute with China, but because Democrat Seth Moulton, a Massachusetts congressman who never gain steam in his presidential campaign, had dropped out.

"The Dow is down 573 points perhaps on the news that Representative Seth Moulton, whoever that may be, has dropped out of the 2020 Presidential Race!" Trump tweeted.