NEW YORK, United States -- Taiwanese Vice President William Lai vowed on Sunday "to resist annexation" in a speech given during a weekend trip to the United States that China has opposed.

Lai is the frontrunner in Taiwan's presidential elections next year, and is making transit stops in the United States en route to and from Paraguay, where he will attend the inauguration of president-elect Santiago Pena.

His trip has already roused anger from Beijing, which claims democratic Taiwan as part of China, and which particularly dislikes Lai, accusing him of espousing separatist views.

At a New York luncheon on Sunday, Lai reiterated he would continue to uphold the core tenets of the administration of President Tsai Ing-wen -- also hated by Beijing for refusing to accept that Taiwan is a part of China.

"I will continue... our commitment to resist annexation or encroachment upon our sovereignty," he said in a speech carried by Taiwanese channel TTV News.

China had said earlier on Sunday that it was closely monitoring "troublemaker" Lai's visits, and vowed to take "resolute and forceful measures to safeguard national sovereignty".

In a video released by Taiwan's presidential office, Lai also reiterated that they "must uphold the value of democracy and freedom".

"At this critical moment, we promise again here and now that no matter how great the threat of authoritarianism is to Taiwan, we will never be afraid or back down," Lai told an audience that included representatives of the American Institute in Taiwan -- the de facto US embassy for the island.

Taiwan had sought to downplay Lai's US trip as routine -- he is expected to continue on to Paraguay, then stop in San Francisco on his way back to Taiwan.

China's Taiwan Affairs Office on Sunday accused him of "going to the US to betray Taiwan for his own selfish personal gain."

"William Lai continues to stubbornly defend the idea of Taiwanese independence... in an attempt to wrangle votes," said spokeswoman Zhu Fenglian.

"'Taiwan independence' is incompatible with peace and stability across the Taiwan Strait."

In recent years, China has stepped up military and political pressure on Taiwan, conducting near-daily incursions by warplanes and naval vessels around the island.

In April, when Taiwanese President Tsai met US House Speaker Kevin McCarthy in California, China staged three days of military exercises simulating a blockade of Taiwan.