The rising tensions between the world's two biggest economies unnerved investors already on edge on Friday.
Powell offered little by way of concrete assurance, saying the Fed would "act as appropriate to sustain the expansion" - a phrase he returns to frequently - and cautioned that there are "no recent precedents" to guide policy response to the trade war.
"Sadly, past administrations have allowed China to get so far ahead of fair and balanced trade that it has become a great burden to the American taxpayer", Trump further tweeted.
"I have no idea how the president thinks he can order companies to stop working with China". The Office of the U.S. Trade Representative also said existing tariffs on another $250 billion in Chinese imports would go from 25% to 30% October 1 after receiving feedback from the public.
The tariffs would take place in two steps, just as the USA said it would do earlier this month in imposing 10% tariffs on 300 billion dollars (£242 billion) of Chinese goods. Tariffs on some products would take effect on September 1 and others on December 15. With existing general duties on autos taken into account, the total tariff charged on US -made cars would be as high as 50%.
President Donald Trump speaks with reporters as he walks to Marine One on the South Lawn of the White House, Friday, Aug. 23, 2019, in Washington.
It was an "unprecedented statement for a president to make to private business when there is no national security issue involved", he said.
"Where does this end?" said David French, a senior vice president for the National Retail Federation.
"The persistent uncertainty shock from the trade war is starting to show up in weaker global data and recession indicators are starting to flash yellow", BofAML team writes in a client note.
Once an emergency is declared, the law gives Mr Trump broad authority to block the activities of individual companies or even entire economic sectors, former federal officials and legal experts said.
"He's not taking this stuff lightly, but he's in a fine mood and looking forward to the G7".
A container ship is docked a port in Qingdao in eastern China's Shandong province.
The country's Commerce Ministry said it would impose additional tariffs on a total of 5,078 American products including agricultural products, crude oil and small aircraft.
"China is signaling yet again that it has no intention of backing off from the trade war, further reducing the likelihood of the US and China agreeing on any sort of trade deal in the coming months", said Cornell University economist Eswar Prasad, former head of the China division at the International Monetary Fund.
The latest salvo in the raging US-China trade war ups the ante after Beijing applied duties of its own to $75 billion in American goods, sending US markets into a tailspin.
For many products sold in the United States, there are few alternatives to Chinese production, and shifting production for major goods produced there could take years and be expensive.
Losses on Wall Street were initially muted - in part because stocks were set to open higher on Friday prior to China's announcement. He could also invoke the International Emergency Economic Powers Act, according to the paper or try to punish companies in other ways such as procurement contracts.