China has abandoned its conciliatory tone toward Donald Trump, warning it is prepared to retaliate if the president-elect supports Taiwan independence as a bargaining chip to extract trade concessions.

China considers Taiwan part of its indivisible territory. If Mr. Trump will not respect that, "bilateral co-operation in major fields would be out of the question," foreign ministry spokesman Geng Shuang said Monday, and "there would not be sound, steady growth" of the U.S.-China relationship.

Mr. Geng urged Mr. Trump to "deal with the Taiwan question in a prudent manner so as not to disrupt or damage" the interests of the two countries, or risk the "peace and stability of the Asia Pacific and beyond."

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It was China's strongest warning to date of its extraordinary sensitivity on the Taiwan issue, which observers say Mr. Trump is escalating in ways that risk new tensions between the world's top two economic powers.

Taiwan is "not something tradable," said Chu Shulong, director of the Institute of International Strategic and Development Studies at Tsinghua University in Beijing. If Mr. Trump persists, "China will act firmly. China can do a lot in bilateral relations, in regional and global affairs. I don't think the U.S. will like it."

We have reached "the very important start of a new game between China and the U.S.," said François Godement, director of the Asia and China program at the European Council on Foreign Relations. "This is serious."

China initially sought to quietly persuade Mr. Trump to reconsider after a call with Taiwanese president Tsai Ing-wen on Dec. 2, dispatching with nearly four decades of diplomatic non-contact between leaders of the U.S. and Taiwan.

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Beijing first blamed Taiwan for playing a "little trick." It then dispatched its senior-most diplomat, State Councillor Yang Jiechi, to New York, where he met late last week with Michael Flynn, the retired general Mr. Trump has chosen as his national security adviser. Chinese foreign policy experts voiced optimism that Mr. Trump had merely been misinformed, and blundered into a minefield he would soon leave.

But on Sunday, Mr. Trump said he knew what he was doing — and intended to continue provoking China in hopes of extracting concessions.

"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," he said in a Fox News interview.

He mentioned border tax issues, China's construction of "a massive fortress" in disputed maritime waters and his view that Beijing has not done enough to counteract North Korea's nuclear threat. "They're not helping us at all," Mr. Trump said. "So, I don't want China dictating to me."

Taiwan is a self-governing territory that operates much like an independent country. But Beijing has insisted on international respect for its claim over the region, saying that to question the idea of Taiwan as part of "one China" is to question the sovereignty of China itself.

If Mr. Trump were to fully support Taiwan independence — and it's not clear he will, since any move in that direction would also encounter strong resistance at home — "China would have no grounds to partner with Washington on international affairs and contain forces hostile to the U.S.," the Communist Party-controlled Global Times warned in an English-language editorial published Monday. "In response to Trump's provocations, Beijing could offer support, even military assistance to U.S. foes."

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It added: "Those who advocate Taiwan independence will tremble. … Nothing is impossible if the Trump administration goes too far."

The nationalist paper is prone to inflammatory rhetoric, and does not always reflect the official Chinese position.

But the strength of its language is a window into Chinese disquiet as Mr. Trump enters what Yanmei Xie, China policy analyst at Gavekal Dragonomics, called "very new, very uncharted territory." At first, China was prepared to ignore him. Now it is "taking his words and actions seriously."

Where this might lead is a question that has begun to consume experts in foreign policy and international affairs, many of whom say Taiwan is a dangerous choice as a bargaining chip against China, which sees no room for negotiation on the issue, save a desire to strengthen its one-China policy.

Mr. Trump "is trying to coerce China into submission," Ms. Xie said. "We still want to be optimistic and say both sides know the consequences of a rapidly deteriorating China-U.S. relationship could be disastrous for both countries and for the world. But so many of our assumptions have been upended in the past few months. So I really don't know what to say."

Mr. Godement sees two risks to Mr. Trump's strategy. "One is getting to real conflict, and the cost could be huge. The other is having to backtrack, which would demonstrate the Chinese and Russian belief that the American-led world order is in decline."

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Mr. Trump is not yet president, which means Chinese officials have time to be patient, in hopes a more moderate leader emerges after he takes over the White House, said J. Michael Cole, Taipei-based Senior Non Resident Fellow with the China Policy Institute, University of Nottingham.

But even if the president-elect does act to cool tensions with China once he reaches office, his willingness to employ Taiwan as a pawn carries its own risks.

Taiwan's 23-million people — a population the size of Australia — "should never be a means to an end," Mr. Cole said.

"If Taiwan were indeed used as a bargaining chip, it would be against their will and ultimately would contribute to greater instability in the Taiwan Strait, not to mention loss of faith among other U.S. allies as to the lasting power of its security commitments."