US President Donald Trump says he will impose a 10 per cent tariff next month on the remaining $US300 billion in Chinese imports he has not already taxed, which include a vast range of consumer products like phones and toys.

Key points: The US already has a 25 per cent tariff on $US250 billion of mainly industrial goods

The US already has a 25 per cent tariff on $US250 billion of mainly industrial goods The new tariffs next month are expected to increase the prices of consumer products like iPhones in the US

The new tariffs next month are expected to increase the prices of consumer products like iPhones in the US The move breaks a ceasefire in the US-China trade war, with Mr Trump complaining China is "not moving fast enough"

The move immediately sent stock prices sinking.

Mr Trump has already imposed 25 per cent tariffs on $US250 billion ($367.7 billion) in Chinese products, and Beijing has retaliated by taxing $US110 billion ($161.8 billion) in US goods.

Domestic consumers will likely feel the pain if Mr Trump proceeds with the new tariffs on September 1.

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His earlier tariffs had been designed to minimise the impact on ordinary Americans by focusing on industrial goods, but the new tariffs will hit American consumer products manufactured in China like iPhones, shoes and toys.

The President's announcement via Twitter came as a surprise, in part because the White House on Wednesday (local time) had said Beijing confirmed it planned to increase its purchases of American farm products.

That word came just as US and Chinese negotiators were ending a 12th round of trade talks in Shanghai, which the White House called "constructive," although the negotiations concluded without any sign of a deal.

Negotiations are scheduled to resume next month in Washington.

The Dow Jones Industrial Average, which had been up nearly 300 points earlier in the day, was down nearly 200 points after Mr Trump's tweets announcing the new tariffs.

The Dow closed for the day down 280 points — more than 1 per cent.

Mr Trump has long said he was preparing to tax the $US300 billion in additional Chinese goods, but he had suspended the threat after meeting with President Xi Jinping in Osaka, Japan, in June.

Beijing failed to follow through on promises: Trump

Mr Trump has already imposed 25 per cent tariffs on $US250 billion of Chinese products. ( Reuters: Kevin Lamarque )

It is not clear when American consumers are likely to feel the impact of the additional tariffs, but higher prices could show up in stores from the end of September.

Besides announcing the additional tariffs on Chinese imports, Mr Trump tweeted that "we look forward to continuing our positive dialogue with China on a comprehensive trade deal, and feel that the future between our two countries will be a very bright one!"

The President accused Beijing of failing to follow through on stopping the sale of the drug fentanyl to the US or on purchasing large quantities of US farm goods such as soybeans.

Speaking to reporters on Thursday (local time) at the White House, Mr Trump complained that Mr Xi is "not moving fast enough".

He said he scheduled the additional tariffs to begin on September 1 to give exports already en route from China time to get to the US — a journey that can take three or four weeks.

By setting the import taxes at 10 per cent, he has leeway to ratchet them higher if necessary to further increase pressure on Beijing.

"Until such time as there's a deal … we'll be taxing them," Mr Trump said.

'Potential escalation in trade dispute'

The world's two biggest economies are locked in a trade war over US allegations that Beijing uses predatory tactics — including stealing trade secrets and forcing foreign companies to hand over technology — in a drive to overtake American technological dominance.

Talks had broken down in May after the US accused the Chinese of reneging on earlier commitments.

Australian Trade Minister Simon Birmingham described the new tariffs as a "disappointing potential escalation in the trade dispute".

"There may be some short-term opportunities were these tariffs to be increased as President Trump has announced because it could shift purchasing decisions elsewhere around the globe," Senator Birmingham told the ABC.

"But overall, this type of action has proven to hurt the global economy."

AP/ABC