Any interference with the “one China” policy would impact peace and stability in the Taiwan Strait and hamper Beijing’s relations with the U.S., China’s Taiwan Affairs Office said Wednesday. The comments were in response to President-elect Donald Trump who said Washington DC did not have to agree with the policy which has formed basis of diplomatic relations between the two nations since 1979.

“Upholding the ‘one China’ principle is the political basis of developing China-U.S. relations, and is the cornerstone of peace and stability in the Taiwan Strait,” An Fengshan, a spokesman for China’s policy-making Taiwan Affairs Office, said.

“If this basis is interfered with or damaged then the healthy, stable development of China-U.S. relations is out of the question, and peace and stability in the Taiwan Strait will be seriously impacted,” he warned.

According to the “one China” policy, there is only a single state called ‘China’ despite there being two governments. People’s Republic of China, which is the mainland China most of us know, considers Taiwan (officially known as Republic of China) a renegade province. However, Taiwan considers itself an independent country. Both PRC and Taiwan claim to be the legitimate government of one China, but in reality, Taiwan has control only over a few small islands.

The U.S. has formal diplomatic relations with only PRC. However, it maintains non-official relationship with Taiwan, including continued arms sales to the island nation.

In an interview on “Fox News Sunday,” Trump said: “I fully understand the ‘one China’ policy, but I don’t know why we have to be bound by a ‘one China’ policy unless we make a deal with China having to do with other things, including trade.”

The statement angered China, which was already upset with Trump’s earlier telephone call with Taiwan President Tsai Ing-wen. Beijing is suspicious of Tsai and her ruling Democratic Progressive Party which has pushed for the country’s formal independence.

In a strongly-worded editorial Monday, state-run Global Times said the “one China” policy was not for “selling.”

“Trump thinks that everything can be valued and, as long as his leverage is strong enough, he can sell or buy… Trump needs to learn to handle foreign affairs modestly, especially the China-U.S. relationship. More importantly, a hard struggle against Trump is needed to let him know that China and other world powers cannot be easily taken advantage of,” the newspaper said.