WASHINGTON (REUTERS) - President Donald Trump said on Thursday (April 5) he had instructed US trade officials to consider US$100 billion (S$131.66 billion) in additional tariffs on China, fuelling an already heated trade dispute between the world's two biggest economies.

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

He added that the US Trade Representative had determined that China "has repeatedly engaged in practices to unfairly obtain America's intellectual property".

"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.

US stock futures slid 1 per cent and the dollar dipped against other major currencies, while Asian shares flitted in and out of positive territory. Chinese markets were closed for a holiday.

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

Earlier this week, the Trump administration proposed a 25 per cent tariff on about US$50 billion of US imports, covering 1,300 Chinese industrial and other products. China shot back with a list of similar duties on American imports including soybeans, planes, cars, beef and chemicals.

"Rather than remedy its misconduct, China has chosen to harm our farmers and manufacturers," Mr Trump said.

The tit-for-tat tariffs have stirred fears that the two countries will spiral into a trade war that will crush global growth. Republican lawmakers from Western and Midwestern states have voiced worries about a big hit to US farming exporters.

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.

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

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

Republican lawmakers from Western and Midwestern 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 lawmaker 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 told Reuters, but did not offer specifics.

The US tariffs are aimed at forcing changes to Chinese government policies designed to transfer US intellectual property to Chinese companies.

The USTR's "Section 301" investigation authorising the tariffs alleges that China has systematically sought to misappropriate US intellectual property through joint venture requirements that often cannot be negotiated without technology transfers, something China denies.

A USTR spokeswoman did not immediately respond to a request for more information about which products would be targeted or the selection method and timing of the additional tariffs.

A further US$100 billion in tariffs on Chinese goods would likely expand the scope of Mr Trump's attack to more consumer goods.

The first round of US$50 billion in tariffs mostly targeted industrial goods and electronic components.

The two biggest categories of US imports from China last year were communications and computer equipment, totalling US$137 billion according to US Census data. Cellphones and computers, key portions of these categories, were spared from the initial tariffs list.

Apparel and footwear, both labor-intensive industries in China, made up a combined US$39 billion in US imports.

A USTR spokeswoman said that the US$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.

The new list will be proposed in the same manner as the first round and will go through a similar public comment period before any of them are activated, the spokeswoman said.