China’s Foreign Ministry spokesman Wang Wenbin

China says it has filed a formal complaint with the United States over a visit by US Undersecretary for Economic Affairs Keith Krach to Taiwan, the highest-ranking State Department official to visit Taipei in four decades, in open defiance of China, which has sovereignty over the island.

Beijing filed stern representations with Washington and would respond as needed depending on the situation, China’s Foreign Ministry spokesman Wang Wenbin said in a press conference on Thursday, without giving details.

China firmly opposes any official exchanges between the US and Taiwan; and under the internationally-recognized One China policy, almost all world countries - including the US - recognize Chinese sovereignty over the self-ruled island.

Although Washington has no formal relations with Taipei, it is the island’s largest weapons supplier and an avid backer of Taiwan’s secessionist president Tsai Ing-wen.

China’s reaction came just a day after US State Department’s spokeswoman Morgan Ortagus said that Krach would arrive in Taiwan late on Thursday afternoon.

She added that the US official was heading to Taiwan to attend a memorial service for late president Lee Teng-hui - who died on July 30 - due to be held on Saturday.

The United States honors President Lee’s legacy by continuing our strong bonds with Taiwan and its vibrant democracy through shared political and economic values, Ortagus said.

According to a statement by Taiwan’s foreign ministry, Krach, who is accompanied by Assistant Secretary Robert Destro, would also discuss how to strengthen bilateral economic cooperation during his three-day visit.

The ministry also described Krach as the highest-ranking State Department official to visit the island since 1979, when the US recognized Chinese sovereignty over Taiwan.

Taiwan’s president Tsai will host a dinner for the US delegation on Friday. Her office said in a statement that Taipei looked forward to further exchanges and discussions between Taiwan and the US in a bid to solidify the foundation for more collaborations, including economic cooperation, through... Krach’s visit.

Last month, Washington angered Beijing when US Health Secretary Alex Azar visited Taiwan and met with Tsai.

The visit took place despite warnings from Beijing, which views the self-ruled island as a breakaway province that should be reunited with the mainland.

The new visit by the US official to Taiwan will further strain the already-troubled relations between Washington and Beijing, which are at loggerheads over a host of issues, including trade, Hong Kong, the origins and handling of the COVID-19 pandemic, and the disputed South China Sea.

Relations between the two world powers have hit the lowest level in decades under US President Donald Trump.