China says it has lodged a complaint against the United States over a reported plan for talks next month between Taiwan's President Tsai Ing-wen and US House Speaker Kevin McCarthy.

Britain's Financial Times newspaper on Monday reported that Tsai and McCarthy agreed to meet in the US state of California in April. The newspaper said Tsai plans to stop in the US during a trip to Belize and Guatemala early in the month.

Chinese Foreign Ministry spokesperson Mao Ning told a news conference on Wednesday that China is seriously concerned about the transit plan. She said China "lodged solemn representations with the US side and asked them to clarify."

Mao reaffirmed the Chinese government's stance, which describes Taiwan issues as at the very core of China's core interests. She said Taiwan is "the first red line" that must not be crossed in China-US relations.

Mao also warned that China strongly objects to any form of official exchange between the US and Taiwan, and any form of contact with what China calls Taiwan's independence forces.