China said Monday it had "serious concern" about president-elect Donald Trump's most recent comments concerning Taiwan, and warned that any changes to how the U.S. deals with the self-governing island could damage diplomatic ties between Washington and Beijing.

China's comments came a day after Trump said on Fox News Sunday with Chris Wallace that he didn't feel "bound by a one-China policy."

Geng Shuang, a spokesman for China's Foreign Ministry, said that established policy is the "political foundation" of any diplomatic relationship between China and the U.S., and that any damage to it could render co-operation between both sides to be "out of the question."

Many people might be surprised at how the new U.S. leader is truly a 'businessman' through and through. But in the field of diplomacy, he is as ignorant as a child. - Communist Party's Global Times

"We urge the new U.S. leader and government to fully understand the seriousness of the Taiwan issue, and to continue to stick to the one-China policy," Geng said.

Since recognizing the People's Republic of China in 1979, the U.S. has adhered to the one-China policy, recognizing Beijing as the capital of China and maintaining only unofficial relations with Taiwan. U.S. law, however, requires the U.S. to ensure Taiwan has the means to defend itself and to treat all threats to the island as matters of serious concern.

China regards the island as a breakaway province to be reunified with the mainland — by force, if necessary.

Geng's comments are the strongest public condemnation China has made of Trump's criticisms of current American policy toward Taiwan.

Beijing was already angered by Trump's Dec. 2 phone call with Taiwanese President Tsai Ing-wen, the first time a U.S. president or president-elect has publicly spoken to a Taiwanese leader in nearly four decades. China considers any reference to a separate Taiwanese head of state to be a grave insult.

Provocative tweets after Taiwan call

Trump followed the call with two tweets accusing China of manipulating its currency, unfairly taxing American imports and provoking tensions in the South China Sea.

On Sunday, he told Fox News Sunday that he wouldn't feel "bound by a one-China policy unless we make a deal with China having to do with other things, including trade."

Trump said his call with Tsai was "very nice" and strictly meant to congratulate him on his winning the presidential election.

"Why should some other nation be able to say I can't take a call?" he said. "I think it actually would've been very disrespectful, to be honest with you, not taking it."

Hours after the interview aired, China's Communist Party-controlled Global Times published a Chinese-language editorial headlined: "Trump, please listen clearly: 'One China' cannot be traded."

"China needs to launch a resolute struggle with him," the editorial said. "Only after he's hit some obstacles and truly understands that China and the rest of the world are not to be bullied will he gain some perception."

"Many people might be surprised at how the new U.S. leader is truly a 'businessman' through and through," the paper said, referring to Trump's suggestion of using the one-China policy as a bargaining chip. "But in the field of diplomacy, he is as ignorant as a child."

The Global Times, which is published by the Communist Party's mouthpiece, the People's Daily, often runs commentaries that target nationalistic sentiment with provocative language.

On Monday afternoon, the White House said the United States is committed to its longstanding policy that Taiwan is part of "one China," but will not use Taiwan as leverage in any dealings with Beijing.

White House spokesman Josh Earnest was asked about Trump's comments made a day earlier that he did not see why the United States must be bound by the policy unless it gets something in return from China.

"The United States does not view Taiwan and our relationship with Taiwan as a bargaining chip. Taiwan is not a source of leverage; it's a close partner of the United States," Earnest told a news briefing. "And bargaining that away is not something that this administration believes is our best interest."