The order follows US President Donald Trump's imposition of additional 10 percent tariffs on Chinese imports starting Tuesday.

Minutes after they took effect, Beijing unveiled new levies on a wide range of US goods in retaliation, fuelling fears of an escalating trade war between the world's two largest economies.

The USPS halt, which was unveiled over the weekend, will take place "until further notice".

Asked about the suspension at a regular news briefing on Wednesday, China's foreign ministry said that "as a matter of principle... we urge the United States to stop politicising trade and economic issues and using them as tools, and to stop the unreasonable suppression of Chinese companies".

Spokesman Lin Jian added that China would "continue to take necessary measures to resolutely safeguard the legitimate rights and interests of Chinese companies".

The order also eliminated a duty-free exemption for low value packages.

The "de minimis" exemption allows goods valued at $800 or below to come into the United States without paying duties or certain taxes.

But it has faced scrutiny because of a surge in shipments claiming the exemption in recent years.

US officials pointed to the growth of Chinese-founded online retailers Shein and Temu as a key factor behind this increase -- and Tuesday's halt could delay parcels from both companies from entering the country.

Washington has been looking to tighten the rule, saying the growth in shipments makes it harder to screen goods for security risks.

However, the USPS gave no reason for its pause on Tuesday.

AFP has reached out to Shein and Temu for comment.

Other retailers such as Amazon might also be impacted.

A postal services worker at the Macau General Post Office told AFP on Wednesday that parcels could still be sent to the United States from the special administrative region.

"For now we are not affected, because (the suspension) only affects parcels from mainland China and Hong Kong," he said.

AFP has contacted the Macau postal service for further comment.