US President Donald Trump has tripled tariffs on small parcels exported from mainland China and Hong Kong which were exempt from duties, amid an escalating crossfire between the world’s two biggest economies.

An amendment on reciprocal tariffs published by the White House on Wednesday said the United States would tax imports of items priced up to US$800 at the rate of 90 per cent of their value, up from the original levy of 30 per cent.

Washington will also increase the per postal item fee on goods entering after May 2 and before June 1 to US$75 from the planned US$25. Parcels entering after June 1 will pay US$150 per item.

While the tax exemption on items of marginal value under a rule dating back to the 1930s was due to end on May 2, the latest increase comes after Beijing retaliated against Trump’s previous round of tariffs.

Chinese online marketplaces such as Temu and Shein had taken advantage of the de minimis loophole to enter the US duty free so far.

The increases come after Beijing hit back at the sweeping tariffs announced by the US last week, saying it would impose a 34 per cent levy on imported American goods, in addition to existing duties, in retaliation to similar moves by the US.