The US is "opening fire" on the world with its threatened tariffs, China has warned, saying no one wants a trade war but it will respond the instant US measures go into effect.
The Trump administration's tariffs on $US34 billion ($A46 billion) of Chinese imports are due to go into effect at on Friday.
US President Donald Trump has threatened to escalate the trade conflict with tariffs on as much as $US450 billion worth of Chinese goods if China retaliates, with the row roiling financial markets including stocks, currencies and the global trade of commodities from soybeans to coal.
China has said it will not "fire the first shot", but its customs agency made clear on Thursday that Chinese tariffs on US goods would take effect immediately after US duties on Chinese goods kick in.
Speaking at a weekly news conference, Commerce Ministry spokesman Gao Feng warned the proposed US tariffs would hit international supply chains, including foreign companies in the world's second-largest economy.
"If the US implements tariffs, they will actually be adding tariffs on companies from all countries, including Chinese and US companies," Gao said.
"US measures are essentially attacking global supply and value chains. To put it simply, the US is opening fire on the entire world, including itself," he said.
"China will not bow down in the face of threats and blackmail and will not falter from its determination to defend free trade and the multilateral system."
Asked whether US companies would be targeted with "qualitative measures" in China in a trade war, Gao said the government would protect the legal rights of all foreign companies in the country.
"We will continue to assess the potential impact of the US-initiated trade war on companies and will help companies mitigate possible shocks."
Gao said China's foreign trade was expected to continue on a stable path in the second half of the year, though investors fear a full-blown Sino-US trade war would deal a body blow to Chinese exports and its economy.
Foreign companies accounted for $US20 billion, or 59 per cent, of the $US34 billion of exports from China that will be subject to new US tariffs, with US firms accounting for a significant part of that 59 per cent, Gao said.
China's plans to impose tariffs on hundreds of US goods targets some top US exports, including soybeans, sorghum and cotton, threatening US farmers in states that backed Trump, such as Texas and Iowa.
Chinese buying of soybeans has ground almost to a halt ahead of the duties, while Chinese farmers worry the penalties and tighter supplies will drive up costs, squeeze margins and ultimately inflate retail prices of pork, the country's top-selling meat.
A US industry source said: "There is a 99 per cent chance that tariffs go into force on Friday."
A senior Western diplomat told Reuters there was no sign of any talks at the moment between the two countries, even via back channels.
