The Trump administration will begin withdrawing from a United Nations pact that offered low rates for foreign postal deliveries of small packages in the United States, the latest move to challenge practices it sees as unfairly advantageous to China.

White House officials said on Wednesday the United States would start the process of leaving the Universal Postal Union (UPU), a Switzerland-based organisation that connects postal services worldwide.

The White House said the UPU enables foreign postal services to take advantage of cheap shipments to the United States, creating an unfair cost advantage over American companies that ship goods, and hurting the US Postal Service.

Shares of internet-based mailing and shipping provider Stamps.com fell nearly 9 percent after the announcement. Online shoppers in the United States have often benefited from the arrangement, gaining access to foreign goods at little cost.

President Donald Trump is distancing the United States from international multilateral organisations and accompanying policies that he says hurt American interests. Mr Trump has announced US withdrawal from the Paris agreement to mitigate climate change and an international deal with Iran to curb Tehran's nuclear programme.