Beijing issued its first clear warning Monday, December 12 over Donald Trump's fiery rhetoric, as state media said the Asian giant could back "forces hostile to the US" if the president-elect follows through with threats to drop Washington's One China policy, AFP reports.

It was the strongest signal yet from Chinese authorities that abandoning the One China policy, which guides relations with self-ruling Taiwan, would upset decades of carefully managed Sino-US relations and end cooperation between the world's top two economies.

Beijing has not controlled Taiwan for more than 60 years but foreign ministry spokesman Geng Shuang said it considered the island a "core interest" that affected China's sovereignty and territorial integrity.

The One China policy was the "political bedrock" for relations with the US, he added, and if it was "compromised or disrupted", sound and steady growth in China-US relations and cooperation in major fields would be "out of the question", he told reporters.

The comments came in response to Trump's remarks in an interview Sunday that he did not see why Washington must "be bound by a One China policy unless we make a deal with China having to do with other things, including trade".

He vehemently defended taking a call earlier this month from Tsai Ing-wen, the democratically elected president of Taiwan, which Beijing regards as a rogue province awaiting unification.

Although the United States is Taiwan's main ally and arms supplier, Washington has not had official diplomatic relations with Taipei since 1979, when it switched recognition to Beijing.

If the US openly supports Taiwan's independence and ramps up arms sales to the island, it threatened, China could aid "forces hostile to the US."

"In response to Trump's provocations, Beijing could offer support, even military assistance to US foes," it said.

"China would introduce a series of new Taiwan polices, and may not prioritise peaceful reunification over a military takeover."