With one phone call to the Taiwanese president on Friday, President-elect Donald Trump became the first American president to have officially and publicly spoken directly with Taiwan's leader since 1979.

The move, seen as the first foreign policy incident for Mr. Trump even before he officially takes office next year, has already prompted a formal complaint from China. Beijing still considers Taiwan as part of China's territory, and Trump's move raises concerns about what this means for the US-China relations. Tsai-Ing Wen, the current Taiwanese president, belongs to a political party that espouses independence from China.

“The President of taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you!” Trump tweeted Friday evening. He followed up with another tweet after receiving a wave of criticism. “Interesting how the US sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call.”

The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you! — Donald J. Trump (@realDonaldTrump) December 3, 2016

For a short 10-minute call to ignite such a diplomatic furor reflects the longstanding tensions between China, Taiwan and the United States. China views the overture as a move staged by Taiwan to gain recognition as a separate nation, and Trump’s acceptance of the call as an affront to Sino-US ties. Trump’s unorthodox move with Taiwan is also seen within the context of his harsh rhetoric toward China during the presidential campaign.

In an editorial, the Chinese state-run newspaper Global Times blamed Taiwan, not Trump. It called the call a “petty gesture” by Taiwan to engage Trump, who is “not familiar with foreign relations and has been known for not playing by the rules."

“The Sino-US relationship was formed by the accumulation of interactions between the two societies over the past decades,” the paper wrote. “If Trump wants to overstep the One-China principle, he will destroy Sino-US ties. That means the current pattern between Beijing and Washington as well as international order will be overturned. We believe this is not what Trump wants.”

In a formal protest to the White House, the Chinese foreign ministry said in a statement “it must be pointed out that there is only one China in the world,” as reported by the Financial Times.

The US officially broke diplomatic ties with Taiwan in 1979 when it prioritized diplomatic recognition of China over Taiwan, although it still maintains close economic relations with the island. After the 1949 civil war, Taiwan became home to a group of Chinese nationalists who fled to the island, thus setting the stage for decades of tense relations, including occasional military action.

Ms. Tsai was elected president earlier this year after eight years of rule by a pro-China administration. Her success reignited the friction between China and Taiwan, with Beijing making small but clear threats to create economic problems for Taiwan because Tsai won't accept that Taiwan belongs to China.

“Regardless if it was deliberate or accidental, this phone call will fundamentally change China’s perceptions of Trump’s strategic intentions for the negative,” Evan Medeiros, former Asia director at the White House national security council and current head of Asia research at Eurasia Group told the Financial Times. “With this kind of move, Trump is setting a foundation of enduring mistrust and strategic competition for US-China relations.”

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According to statements from Trump’s transition team and Taiwan’s presidential office, the conversation did not touch on changes to any US-Taiwan formal ties or establishment of direct communication in the future. They mainly discussed the regional situation in Asia, strengthening of bilateral relations and issues of governance.

Reports by The Guardian and local paper Taiwan News say that a Trump representative had visited a Taiwanese city in mid-November to scout business opportunities in the area.