US President Donald Trump has postponed measures to close a tariff exemption on low-cost shipments from China while officials figure out how to tax the millions of packages that arrive in the US every day, reports London's Financial Times.

Amending the enabling executive order, the White House said the de minimis clauses - which exempt shipments under US$800 in value from tariffs and rigorous customs checks - would remain until "adequate systems are in place to fully and expediently process and collect tariff revenue."

Trump had cancelled the exemption in an executive order last week that imposed an additional 10 per cent tariff on goods from China, which the White House said was aimed at punishing Beijing for allowing the flow of deadly opioid fentanyl into the US.

The rest of the tariffs remain in effect. China retaliated, announcing tariffs of 10 to 15 per cent on US liquefied natural gas, coal, crude oil and farm equipment.