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Beijing has warned the incoming US administration that any attempt to challenge the "One China" policy could affect peace in the Taiwan Strait.
Interference may also damage developing US-China relations, a spokesman said.
Under the "One China" policy, the US has formal ties with China rather than the island of Taiwan, which China sees as a breakaway province.
However, US President-elect Donald Trump has expressed doubts about continuing to abide by the policy.
Mr Trump had already angered China by taking a phone call from Taiwan's President Tsai Ing-wen, and then tweeting about it.
On Monday, China said it was "seriously concerned" by Mr Trump's comments, and urged sensitivity around the issue.
But An Fengshan, a spokesman for China's policy-making Taiwan Affairs Office, went further on Wednesday, warning of more serious consequences.
'Ready to confront'"Upholding the "One China" principle is the political basis of developing China-US relations, and is the cornerstone of peace and stability in the Taiwan Strait," he said.
"If this basis is interfered with or damaged then the healthy, stable development of China-US relations is out of the question, and peace and stability in the Taiwan Strait will be seriously impacted," he added.
Mr An's comments came as Admiral Harry Harris, head of the US Pacific Command, vowed the US will keep challenging Beijing's "assertive, aggressive behaviour in the South China Sea".
Speaking to Australian think tank the Lowy Institute in Sydney, Admiral Harris said: "We will not allow the shared domains to be closed down unilaterally, no matter how many bases are built on artificial features in the South China Sea.
"We will co-operate where we can but we will be ready to confront where we must."
Beijing has been developing artificial islands capable of hosting military planes in the region.
It also insists on sovereignty over virtually all the resource-endowed South China Sea, despite rival claims from its South East Asian neighbours.
Washington has repeatedly said it does not recognise the claims, and has sent warships into the area to assert the right to freedom of navigation.