China called on the United States to withdraw troops from Taiwan on Friday in response to reports that a small presence of American troops have been training forces there for at least a year.

Chinese Foreign Ministry spokesman Zhao Lijian said the United States should halt military contact with Taiwan and uphold its commitment to the 1979 One-China agreement. Under the pact, the United States agreed to recognize formal ties with China rather than the island of Taiwan.

“The One China principle is the political foundation of China and U.S. relations,” Zhao told reporters on Friday. “The U.S. must sever diplomatic relations and abrogate its mutual defense treaty with Taiwan and U.S. forces must withdraw from Taiwan.”

Zhao made the statements after the Wall Street Journal reported on Thursday that a U.S. special-operations unit and a contingent of Marines have been operating in Taiwan for at least a year.

U.S. officials confirmed the report.

Taiwan President Tsai Ing-wen declined to comment on the matter Friday but acknowledged tensions “could have a devastating effect on international security and the global economy if they are not handled carefully,” The Washington Post reported.

Taiwan conducted a series of military exercises last month during a mock confrontation with China, including testing Taipei’s military response to a simulated biochemical attack.

Taiwan and the United States have expressed concerns about incursions into Taiwanese airspace by Chinese military aircraft. China claims Taiwan as a sovereign territory, but the democratic island considers itself a country.

Taiwanese defense minister Chiu Kuo-cheng said earlier this week he fears China could launch a full-scale invasion by 2025.

A video summit between President Joe Biden and Chinese President Xi Jinping is planned before the end of the year.