China has notified the World Trade Organisation it is imposing $611.5 million worth of retaliatory tariffs on $2.75 billion worth of US imports including pork, nuts and ethanol in response to US duties on aluminium and steel, a WTO document showed.

The document, dated last Thursday but posted only after the Easter public holidays, came after China said late on Sunday it has increased tariffs by up to 25 percent on 128 US products, escalating a dispute between the world's biggest economies.

China has fulfilled its legal duty to notify the WTO and other member states of its retaliatory measures.

China's ambassador to Washington said China would take counter-measures of the "same proportion" and scale if the United States imposed further tariffs on Chinese goods, amid growing fears of an impending trade war.

The US tariffs are expected to target products benefiting from Beijing's "Made in China 2025" industrial development programme, although it may be more than two months before the import curbs take effect, US officials have said.

"China does not provoke a trade war, and doesn't want to fight a trade war, but we also aren't afraid of a trade war," Chinese Foreign Ministry spokesman Geng Shuang told a regular news briefing.

The Section 301 investigation initiated by US President Donald Trump is focused on accusations of theft of intellectual property and forced technology transfer by China, charges Beijing denies.

Cui said China has been bolstering its protection of intellectual property rights.

"China has been strengthening its efforts and strengthening our legal system on this particular issue, and we are making good progress," he said.