Donald Trump's efforts to use Taiwan as a bargaining chip in relations with China have met with fierce resistance from Beijing, which said on Monday it didn't negotiate over matters of sovereignty.
The curt but measured response came after the President-elect questioned whether he should adhere to the "one China policy" if Beijing didn't give ground on trade and North Korea.
In its regular press briefing, a spokesman for China's Foreign Minister said they were "deeply concerned" by Mr Trump's comments.
The "one China" policy is the bedrock of China's relations with the US and other countries, including Australia.
It stipulates that there is "one China" and therefore countries which have formal diplomatic relations with Beijing, don't officially recognise Taiwan.
"The one-China policy is the political foundation for the healthy development of the Sino-US relationship," said the spokesman Geng Shuang.
He said the issue of Taiwan involved China's territorial integrity and sovereignty and was part of its "core interests".
"If this foundation is disturbed, there are no grounds to talk about further developing the healthy and stable relationship in important areas between China and the US."
Mr Trump has been flirting with the idea of closer relations with Taiwan since taking a congratulatory phone call from President Tsai Ing-wen on December 4.
China dismissed the first direct contact between a US President or President-elect and a Taiwanese leader since 1979, as a "little trick" by Taipei.
In the days after the call those around Mr Trump sought to reassure Beijing they respected the "one China policy", even as the President-elect fired off a series of angry tweets directed at China.
Then just as fears of heightened tensions between the world's two largest economies were dying down, Trump doubled down on his comments.
"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," Trump said on "Fox News Sunday.
"I mean, look, we're being hurt very badly by China with devaluation, with taxing us heavy at the borders when we don't tax them, with building a massive fortress in the middle of the South China Sea, which they shouldn't be doing, and frankly with not helping us at all with North Korea," Trump said.
China is the largest donor to North Korea and Mr Trump said Beijing was not "helping us out at all" in efforts to stop the renegade state developing nuclear weapons.
