Taiwan president's plan to stop over in Hawaii, Guam angers Beijing
China insists democratic self-ruled Taiwan is part of its territory and opposes any international recognition of the island.
Lai will depart Saturday for a visit to the Marshall Islands, Tuvalu and Palau -- the only Pacific islands among Taiwan's 12 remaining allies.
It will be Lai's first overseas trip since taking office in May.
Taiwanese government officials have previously stopped over on US soil during visits to the Pacific or Latin America, angering Chinese leaders in Beijing.
Lai will spend two nights in Hawaii and one night in Guam, meeting with "old friends" and "think tank members", a source in the Presidential Office told AFP on the condition of anonymity.
Communist China, which has never governed Taiwan, has sought to erase it from the international stage, blocking it from global forums and pressuring companies to list the island as a "Chinese province" on their websites.
Taiwan competes as Chinese Taipei in international sports events and Beijing bristles when Taipei officials meet with foreign politicians or government representatives.
Chinese foreign ministry spokeswoman Mao Ning said Thursday that Lai's planned visits were "separatist actions".
"We have consistently opposed official exchanges between the United States and Taiwan... and any form of the US condoning and supporting Taiwan independence separatists and their separatist actions," Mao said at a regular press conference.
Wu Qian, a spokesperson for China's defence ministry, also said "the Chinese (military) shoulders the sacred mission of safeguarding national sovereignty and territorial integrity".
"(The military) will resolutely crush all secessionist attempts for Taiwan independence," Wu told a news conference.
Haemorrhaging allies
In recent decades, Taiwan has haemorrhaged allies as they jumped ship to an ascendant China, which has deeper pockets to provide aid and investment.
The United States is Taiwan's most important partner and main security backer but does not recognise Taipei diplomatically.
Lai's predecessor Tsai Ing-wen transited through Hawaii and Guam during her first official visit to Pacific allies in 2017.
Tsai also met then-US House Speaker Kevin McCarthy in California in April 2023 during a Latin America trip, to which Beijing responded with military drills around the island.
Tensions between China and Taiwan have escalated since Lai took office in May.
Lai and Tsai both belong to the Democratic Progressive Party, but Lai has been more outspoken in his defence of the island's sovereignty and Beijing calls him a "separatist".
The dispute between Beijing and Taipei dates back to 1949 when Chiang Kai-shek's nationalist forces lost a civil war to Mao Zedong's communist fighters and fled to Taiwan.
China has refused to rule out the use of force to seize Taiwan and in recent years has ramped up military activity around the island to pressure Taipei into accepting its claims of sovereignty.
Though Taiwan has its own government, military and currency, it has never formally declared independence and lives under the constant threat of invasion by China.
The United States and China have long butted heads over Taiwan, an island of 23 million people, which has evolved into a vibrant democracy and powerhouse in the semiconductor industry.