US President Donald Trump over the weekend launched the opening salvo in an escalating trade war with China, imposing a 10 percent tariff hike on goods coming from mainland Chinese and Hong Kong.

A spokesperson for the financial hub said Friday the Hong Kong government "will formally launch procedures in accordance with the WTO Dispute Settlement Mechanism against the US' unreasonable measures to defend our legitimate rights".

The US tariffs are "grossly inconsistent with the relevant WTO rules and ignore our status as a separate customs territory", the spokesperson said, adding that the government "strongly opposes" the measures.

Mainland China also filed a complaint with the WTO to defend its "legitimate rights and interests", its commerce ministry said.

After reverting to Chinese rule in 1997, Hong Kong has been run as a special administrative region and is classed as a separate customs territory.

It has been a WTO member for three decades.

Hong Kong's secretary for commerce and economic development Algernon Yau said Thursday that the tariffs "are not expected to have a large impact".

Goods exported from Hong Kong to the United States in 2023 were valued at around HK$6.1 billion ($780 million) and made up only 0.1 percent of the city's total exports, Yau added.

City officials have for years tread a fine line by insisting Hong Kong is a separate entity in international trade, but politically an "inalienable part" of China.

The United States removed Hong Kong's special trading privileges in 2020 after Beijing imposed a sweeping national security law on the former British colony to curb dissent.

Trump at the time said in an executive order that Hong Kong was "no longer sufficiently autonomous to justify differential treatment in relation to (China)".