China’s foreign ministry on Sunday condemned a brief U.S. visit by Taiwan Vice President William Lai, saying he was a separatist and “troublemaker through and through” and Beijing would take strong steps to protect its sovereignty.

Lai, front-runner to be Taiwan’s next president at elections in January, arrived in New York late on Saturday for what is officially a transit on his way to Paraguay for the inauguration of its president.

China, which claims democratically governed Taiwan as its own territory, has repeatedly denounced Lai’s trip, which includes another stopover in San Francisco on Wednesday on his way back to Taipei.

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 United States.

“Lai stubbornly adheres to the separatist position of Taiwan independence and is a troublemaker through and through,” the ministry 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.”