Beijing has hit out at the United States for receiving William Lai Ching-te, the current Taiwanese vice-president and leading presidential contender, describing him as a "troublemaker" and accusing Washington of using the island to contain China.
Lai briefly entered New York on Saturday before leaving for Paraguay - one of the island's last remaining diplomatic allies - for the inauguration of President Santiago Pena, who himself spent five days in Taiwan in July.
Three separate official statements condemned the visit, with a foreign ministry spokesman saying on Sunday: "We firmly oppose any visit by Taiwan independence separatists to the US ... under whatever pretext, and we firmly oppose the US government having any form of official contact with the Taiwan region."
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Before returning to Taiwan, Lai will again pause in San Francisco on Wednesday.
Beijing said the stopovers reflect how Taipei's efforts to solicit US support for Taiwanese independence have fuelled cross-strait tensions.
In the past Taiwanese officials have used transits through the US to bolster the island's international presence, most recently when President Tsai Ing-wen met House Speaker Kevin McCarthy in California.
Beijing views such exchanges - as well as visits to the island by numerous US politicians, including McCarthy's predecessor Nancy Pelosi - as extremely provocative and destabilising.
"Lai Ching-te clings stubbornly to the separatist position for Taiwan independence. He is a troublemaker through and through," the spokesperson said, echoing previous accusations that the politician will sell out the island to the US.
Beijing said the stopover was arranged by authorities from both sides for Lai to "engage in political activities in the US".
"We urge the US to ... stop all forms of official interaction with Taiwan, stop conniving at and supporting Taiwan independence separatist forces and their separatist activities and stop fudging and hollowing out the one-China principle," the ministry's statement concluded.
The United States, in common with most countries, does not recognise Taiwan as independent but is opposed to any forcible change in the status quo.
The Chinese consulate general in New York accused the US of using "tricks" to "obscure and hollow out" the one-China principle, and of constantly challenging China's red line by using "salami tactics".