The statement on Sunday came hours after Lai arrived in New York for what was officially billed as a transit stop en route to Paraguay.
Lai, the frontrunner to be Taiwan’s next president at elections in January, is on his way to Paraguay to attend President-elect Santiago Pena’s inauguration and is scheduled to make a second stop in the US city of San Francisco on his way back to Taipei.
China, which claims democratically governed Taiwan as its own territory, has repeatedly denounced Lai’s trip.
In a statement issued shortly after Lai landed in New York on a scheduled flight from Taipei, China’s foreign ministry said it opposed any form of visit by “Taiwan independence separatists” to the US.
“Lai stubbornly adheres to the separatist position of Taiwan independence and is a troublemaker through and through,” it said.
Taiwan is the “core of China’s core interests” and facts have shown again and again that the reason for the rise in tensions in the Taiwan Strait is Taiwan trying to “rely on the United States to seek independence”, it said.
“China is closely following developments and will take resolute and vigorous measures to defend national sovereignty and territorial integrity,” it added.
Lai has been far more outspoken about independence than Taiwanese President Tsai Ing-wen, to whom Beijing is already hostile as she refuses to accept its view that Taiwan is a part of China.
The Harvard-educated doctor-turned-politician has previously described himself as a “pragmatic Taiwan independence worker”, and reiterated this week when speaking with a local television channel that Taiwan was “not part of the People’s Republic of China (PRC)”.
“The Republic of China and PRC are not subordinate to each other,” he said, using Taiwan’s official name.
On landing in New York on Sunday, Lai said on the social media platform X that he was “happy to arrive at the Big Apple, icon of liberty, democracy and opportunities”.
He added that he was greeted at the airport by representatives of the American Institute in Taiwan, the de facto embassy for the island in the US and said he was “looking forward to seeing friends and attending transit programs in New York”.
A person familiar with the trip’s planning told the Reuters news agency that Lai will join receptions with overseas Taiwanese during the stopover in New York, during which he will make a speech.
He will not meet US legislators, the person said, adding Lai will keep the visit “low key”, in line with Taipei and Washington’s shared position to “jointly manage risks when facing an authoritarian region at this sensitive time”.
Taiwanese officials say China is likely to launch military drills this week near Taiwan, using Lai’s US stopovers as a pretext to intimidate voters ahead of next year’s election and make them “fear war”.
On Sunday, the Eastern Theatre Command of China’s People’s Liberation Army, which is responsible for the area around Taiwan, posted on its WeChat account a short video of fighter jets practicing dog fights at an undisclosed location, saying its forces had recently been engaged in “high-intensity flight training”.
“We pay attention to daily training, constructing an actual combat environment and treating every lift-off into the skies as actual combat,” it quoted an unnamed commander as saying.
Lai’s transits come as Beijing and Washington are trying to improve relations.
That includes the prospect of a visit to Washington DC, by Chinese Foreign Minister Wang Yi, which could pave the way for a meeting between US President Joe Biden and China’s leader Xi Jinping this year.
China has stepped up its military activities around Taiwan over the past three years, seeking to force the island into accepting Beijing’s sovereignty.
In April, China staged war games around Taiwan in an angry response to Taiwan President Tsai Ing-wen meeting US House Speaker Kevin McCarthy in California on a stopover on the way back to Taipei following her visit to Central America.