US President Donald Trump said the United States has already lost any trade war, as he defended his proposed tariffs against Chinese goods, saying the move might cause "a little pain" but the United States will be better off in the long run.

"We've already lost the trade war. We don't have a trade war, we've lost the trade war," Mr Trump said in an interview with a New York radio show.

"I'm not saying there won't be a little pain, but the market has gone up 40%, 42% so we might lose a little bit of it. But we're going to have a much stronger country when we're finished.

"So we may take a hit and you know what, ultimately we're going to be much stronger for it," Mr Trump said.

Mr Trump has directed trade officials to identify tariffs on $100 billion more Chinese imports, upping the ante in an already high-stakes trade confrontation between the world's two largest economies.

The further tariffs were being considered "in light of China's unfair retaliation" against earlier US trade actions, which included a proposed $50 billion of tariffs on Chinese goods, Mr Trump said in a White House statement.

"This is what a trade war looks like, and what we have warned against from the start," said National Retail Federation President and CEO Matthew Shay.

"We are on a dangerous downward spiral and American families will be on the losing end," Mr Shay added in a statement, urging Mr Trump "to stop playing a game of chicken with the US economy".

Financial markets, roiled for days by the trade fight and Mr Trump's management of it, whipsawed again on the new threat. After a bullish regular trading day, US equity futures sold off sharply in after-market-hours trading.

Doug Kass, who runs hedge fund Seabreeze Partners Management Inc, added: "Our president is going to make market volatility and economic uncertainty great again."

In his statement, Mr Trump said the US Trade Representative had determined that China "has repeatedly engaged in practices to unfairly obtain America's intellectual property".

The tit-for-tat escalation of tariff announcements have stirred fears that the two countries will spiral into a trade war that will crush global growth.

Chinese state media slammed Mr Trump's threat of more trade action against China as "ridiculous".

"This latest intimidation reflects the deep arrogance of some American elites in their attitude towards China," the state-run Global Times said in an editorial.

On Tuesday, USTR proposed 25% tariffs on more than 1,300 Chinese industrial and other products from flat-panel televisions to electronic components.

China shot back 11 hours later with a list of proposed duties on $50 billion of American imports, including soybeans, aircraft, cars, beef and chemicals.

"Rather than remedy its misconduct, China has chosen to harm our farmers and manufacturers," the Republican president said.

Republican politicians from western and mid-western states have voiced worries about a big hit to US farming exporters.

On the new tariff threat, Republican Senator Ben Sasse said in a statement: "Hopefully the president is just blowing off steam again but, if he's even half-serious, this is nuts.

"Let's absolutely take on Chinese bad behaviour, but with a plan that punishes them instead of us. This is the dumbest possible way to do this," said the politician from the farm-belt state of Nebraska.

The US Department of Agriculture is looking for ways to assist farmers affected by China's tariffs, USDA undersecretary Bill Northey said, but did not offer specifics.

White House officials have suggested throughout the week that talks with the Chinese could help resolve trade issues between the two countries.

China ran a $375 billion goods trade surplus with the United States in 2017. Mr Trump has demanded that China cut the trade gap by $100 billion.

A senior US official said no formal negotiating sessions had yet been set, but that the United States was willing to negotiate with China.

The US tariffs are aimed at forcing changes to Chinese government policies designed to transfer US intellectual property to Chinese companies and allow them to seize leadership in key high-technology industries of the future.

A USTR spokeswoman said that the $100 billion second-round of potential tariffs had not been determined yet and would be selected by USTR career staff - not political appointees - along with economists and trade experts.