US President Donald Trump says he has "hereby ordered" American companies to leave China, after Beijing announced plans to slap new tariffs on US goods.
The White House did not immediately say what authority the president had to compel private firms to quit a country.
The US president also vowed swift retaliation against Beijing after it unveiled plans for duties of 10 percent on $US75 billion of US goods.
The latest salvos in the US-China trade war sent global financial markets tumbling.
The Dow Jones Industrial Average lost more than 620 points, or 2.4 percent, on Friday. The S&P 500 dropped 2.6 percent while the Nasdaq was 3 percent lower.
London's FTSE 100 and the German DAX also turned negative.
In a blistering Twitter screed on Friday, Mr Trump said: "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."
He also tweeted: "We don't need China and, frankly, would be far... better off without them.
"Our Country has lost, stupidly, Trillions of Dollars with China over many years. They have stolen our Intellectual Property at a rate of Hundreds of Billions of Dollars a year, & they want to continue. I won't let that happen!
China's new tariffs will range between 5 percent and 10 percent and apply to more than 5000 goods coming from the US.
Agricultural goods, crude oil and small aircraft were among the targeted items.
Mr Trump tweeted on Friday that he would be "responding to China's Tariffs this afternoon".
"This is a GREAT opportunity for the United States," he wrote.
Beijing also said it would revive a 25 percent tariff on US car imports lifted earlier in 2019 in a goodwill gesture as the two countries tried to negotiate a trade agreement.
Carmakers warned the tax would put US jobs at risk.
"When these tariffs were initially imposed by China in 2017, American exports of finished vehicles dropped by 50 percent," said John Bozzella, who represents car manufacturers. "We can't let that happen to American workers again."
Mr Trump unveiled a 10 percent tariff on $300bn of Chinese goods on 1 August, blaming China for not following through on promises to buy more American agricultural products.
That tariff was expected to be introduced on 1 September, but less than two weeks later Mr Trump delayed that date to 15 December partly due to concerns it might hit Christmas shoppers.
China said it planned to impose its new tariffs in two stages on 1 September and 15 December.
Mr Trump also turned his fire on Friday against the head of the US central bank after he spoke out about the economic risks of a trade war with China, questioning whether Federal Reserve chair Jerome Powell was a greater "enemy" than China's leader Xi Jinping.
At a symposium of central bankers in Jackson Hole, Wyoming, on Friday Mr Powell warned that trade tensions were hitting the global economy.
He also said the Fed - the world's most powerful central bank - did not have a "rulebook" to deal with the fallout.
The White House moved to play down the new tariffs. Mr Trump's trade adviser, Peter Navarro, told CNN that the Chinese duties were "well signalled," adding "this isn't breaking news".
He said that talks between the two countries were on schedule and argued that the tariffs were not hurting Americans. "Consumers aren't feeling the pain [of the trade war] and we are focused on making sure they [China] feel the pain, not us."
He added that the economic slowdown was the fault of the US Federal Reserve, which has drawn criticism from Mr Trump for not making bigger cuts in interest rates.
Mr Navarro said the central bank should lower the benchmark interest rate further to stimulate growth.
US bond markets have recently sent warning signals of an impending recession. However, equities are trading at near-record highs and employment is at its highest level in almost 50 years.
Mr Trump also said on Twitter on Friday that the US economy was "strong and good", while "the rest of the world is not doing so well".
Stock markets in the US fell following news of China's tariffs, but then recovered, only to fall again in response to Mr Trump's tweets.