(CNN)President-elect Donald Trump spoke with Taiwan’s President Friday, threatening to ignite a diplomatic showdown with China even before he takes office.

“President-elect Trump spoke with President Tsai Ing-wen of Taiwan, who offered her congratulations,” Trump’s transition team said in a statement. “During the discussion, they noted the close economic, political, and security ties exists between Taiwan and the United States. President-elect Trump also congratulated President Tsai on becoming President of Taiwan earlier this year.”

Trump’s conversation marks the first publicly reported call between a US President or President-elect and the leader of Taiwan since Washington established diplomatic relations with Beijing in 1979, said Bonnie Glaser, a China expert at the Center for Strategic and International Studies.

The telephone call is certain to incense China, which considers Taiwan a renegade province. It is the first major sign of the unpredictability that Trump has vowed to bring to long-held US relations with the rest of the world.

The call, first reported by the Financial Times, risks throwing US-China relations into a tailspin before Trump takes the oath of office on January 20. China’s state-run CCTV quickly issued a statement saying Trump made “an unprecedented break with the One-China Policy and accepted US-Mainland protocol.”

“There is no immediate reaction from the Chinese government to this call,” the statement continued. “The Mainland says it firmly opposes official contact in any form between Washington and Taipei.”

“The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you!” tweeted the President-elect.

He soon followed up by tweeting, “Interesting how the U.S. sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call.”

Taiwan’s President on Friday night published a statement about the phone call on an official website, which she described as lasting ten minutes.

“During the call President Tsai and President-elect Trump, besides having an intimate and relaxed conversation, also shared their views and concepts on future important policy points,” the statement said, according to a translation. “In particular, to promote the domestic economy and strengthen national defense, allowing the people better lives and a guarantee of security. The two briefly exchanged opinions on the situation in the Asia region.”

Kellyanne Conway, Trump’s campaign manager, told CNN’s Anderson Cooper on “AC360” that she wouldn’t go beyond the call readout from the transition team statement. But the President-elect was fully aware of the call’s implications, Conway suggested.

“He either will disclose or not disclose the full contents of that conversation but he’s well aware of what US policy has been,” Conway said.

By Friday night, China had already reached out to the Obama administration. White House officials declined to comment on diplomatic discussions.

“We remain firmly committed to our ‘one China’ policy based on the three Joint Communiques and the Taiwan Relations Act,” he said. “Our fundamental interest is in peaceful and stable cross-Strait relations.”

A different Obama administration official said there was no contact with either the White House nor the State Department about the call beforehand.

China: The biggest issue for the next US president?
China: The biggest issue for the next US president? 00:46

Stephen Yates, a fellow at the Heritage Foundation who is an adviser to Trump’s transition, is in Taiwan and helped facilitate the call, according to a source familiar with the visit. Yates was an Asia adviser to former Vice President Dick Cheney and is very supportive of Taiwan.

“This could be damaging,” said Barry Pavel, director of the Brent Scowcroft Center on International Security at the Atlantic Council. “We’ll see the Chinese reaction in several hours. I don’t expect it to be pretty.”

The danger, Pavel said, is that “the Chinese are going to think it was deliberate and that this is the beginning of a hostile policy by the Trump administration, upending the basic geometry of diplomatic relations between the US and China since 1979.”

The President-elect vowed in his election campaign to take a tough line toward Beijing. He vowed to brand it a currency manipulator and warned the Asian giant is committing “rape” against American workers with its trade policy.

It was not immediately clear whether Trump’s move in talking to Tsai was a deliberate shot across China’s bows, or whether it was a product of his and his team’s inexperience in foreign policy.

Glaser, the Asia expert at…