China has warned the United States that the long-standing "One-China" policy is nonnegotiable amid signs that President-elect Donald Trump wants to boost relations with Taiwan.

Chinese Foreign Ministry spokesman Lu Kang said in a statement on January 15 that there is "only one China in the world" and that the communist government in Beijing is the "only legitimate government representing China."

He added that Taiwan is "an inalienable region of China."

Trump has voiced his displeasure over what he calls unfair Chinese trade practices that he has suggested could be changed by using the "One-China" policy to bargain with Beijing.

"Everything is under negotiation, including [the] 'One-China' policy,'" he told The Wall Street Journal in a January 13 interview.

Trump first angered Chinese officials by accepting a congratulatory phone call from Taiwanese President Tsai Ing-Wen after he won the November 8 presidential election.

It was the first time a U.S. president or president-elect had spoken with a Taiwanese president since Washington decided to only recognize Beijing in 1979.

The Chinese government considers Taiwan -- which is recognized by 20 countries worldwide -- to be a breakaway province that must be brought under its control.

Tsai, meanwhile, made a controversial stop in San Francisco on January 14 on her way back to Taiwan from South America.

While in California, she stopped at Twitter headquarters, where she set up an account in English. The visit comes after a January 7-8 stop in Texas where she met with Texas Governor Greg Abbott.

Based on reporting by AP and Reuters