China has pushed back against U.S. President-elect Donald Trump’s suggestions that the “one-China” policy on Taiwan is negotiable, with the Foreign Ministry in Beijing urging the incoming administration to “recognize the high sensitivity of the Taiwan question.”

Foreign Ministry spokesman Lu Kang on Saturday called the one-China principle “nonnegotiable” and “the political foundation” of China-U.S. relations.

In an interview with The Wall Street Journal published Friday, Trump again raised the prospect of using Taiwan as a bargaining chip in Sino-U.S. relations.

Asked if he supported the one-China policy on Taiwan, Trump said: “Everything is under negotiation, including one China.”

Washington recognized Beijing diplomatically as the one China in 1979, and has kept only unofficial ties with Taiwan since then — though it has sold arms to Taipei under the Taiwan Relations Act, including some $1.83 billion worth in 2015.

Suggestions in the past that the U.S. may change its stance have been met with alarm in Beijing, including after Trump’s unprecedented phone call with Taiwan President Tsai Ing-wen on Dec. 2. That call was believed to be the first time an American president or president-elect had publicly spoken with Taiwan’s leader since official diplomatic ties were switched to Beijing.

China considers Taiwan a “core interest,” and views the self-ruled island as a renegade province that must be brought back into the fold — by force, if necessary.

In a statement posted to the Chinese Foreign Ministry’s website, Lu called Taiwan “an inalienable part of China. … That is the fact acknowledged by the international community and no one can change.”

“The one-China principle, which is the political foundation of the China-U.S. relations, is non-negotiable. In order to avoid disruption to the sound and steady development of the China-U.S. relations and bilateral cooperation in key areas, we urge relevant parties in the U.S. to fully recognize the high sensitivity of the Taiwan question, approach Taiwan-related issues with prudence and honor the commitment made by all previous U.S. administrations of both parties on adhering to the one-China policy and the principles of the three joint communiques.”

According to the Journal interview, Trump appeared impatient with diplomatic protocols involving China and Taiwan, something that has stoked concern among some in the U.S. foreign policy community who have questioned his understanding of the delicacy surrounding the issue.

“We sold them $2 billion of military equipment last year,” Trump was quoted of saying of U.S. arms sales to Taiwan. “We can sell them $2 billion of the latest and greatest military equipment but we’re not allowed to accept a phone call. First of all it would have been very rude not to accept the phone call.”

Trump has also lashed out against China over what he says are unfair trade and currency practices that are “raping” and “killing” Americans.

Shortly after his phone call with Taiwan’s Tsai, Trump said that he didn’t feel “bound by a one-China policy unless we make a deal with China having to do with other things, including trade.”

In the Journal interview, Trump toned down his rhetoric, saying that he would not label China a currency manipulator on his first day in the White House, but still voiced his displeasure at the alleged practices.

Earlier comments by the incoming U.S. leader have stoked concern of a looming trade war.

Trump had threatened during his campaign to brand China a currency manipulator immediately upon taking office on Friday, and to slap 45 percent tariffs on its exports to the U.S.

“I would talk to them first,” he was quoted by the Journal as saying. “Certainly they are manipulators. But I’m not looking to do that.”

The U.S. president-elect has also taken a hard line against Beijing’s moves in the contested South China Sea, where he said China has built “a massive military complex.”

China has built a series of military outposts on reclaimed islands in the Spratly and Paracel chains of the strategic waterway, near which the U.S. has conducted what it calls “freedom of navigation” patrols. Those operations have incensed Beijing, which claims almost the entire South China Sea, through which about $5 trillion worth of trade passes every year. Brunei, Malaysia, the Philippines, Taiwan and Vietnam all have rival claims there.

In July, the Permanent Court of Arbitration in The Hague rejected China’s expansive claims to much of the waters, a ruling Beijing criticized as “waste paper.”

Experts say Trump’s unorthodox approach to Taiwan — and Sino-American relations in general — has sown uncertainty not only in Beijing, but also in Washington.

“There’s enough confusion in Washington about Trump’s China policy, so you can hardly blame Beijing for wondering what to make of it all. But so far their response to Trump’s highly provocative statements has been measured and restrained,” said David Capie, director of the Centre for Strategic Studies at Victoria University of Wellington.

Still, the measured tone out of Beijing, which has a record of testing U.S. leaders in their initial weeks and months in office, could change once Trump is sworn in this week.

“Beijing has seen plenty of presidents elected on tough anti-China rhetoric only to soften it when they get into power,” said Capie. “But Donald Trump is not your normal president. I think the levels of uncertainty raised in the last month or so are really worrying.”