President-elect Donald Trump held a phone call with Taiwan President Tsai Ing-wen, which raises major concerns for the diplomatic relationship between China and the US.
The US cut diplomatic ties with Taiwan in 1979, as Beijing views the country as a breakaway state.
Financial Times reported that the call was confirmed the Trump transition team. They said that Mr Trum and Ms Tsai "noted the close economic, political, and security ties" between the US and Taiwan.
But it is not clear whether the New York businessman intended to signify a change in US diplomacy. Regardless of Mr Trump's true intentions, the act is likely to provoke a response from China.
"The Chinese leadership will see this as a highly provocative action, of historic proportions," former Asia director at the White House national security council, Evan Medeiros, told the Financial Times.
"Regardless if it was deliberate or accidential," he added, "this phone call will fundamentally change China's perceptions of Trump's strategic intentions for the negative.
"With this kind of move, Trump is setting a foundation of enduring mistrust and strategic competition for US-China relations."
The US first adopted the "One China" policy in 1972 following meetings between then President Richard Nixon and Chairman Mao Zedong. The agreement was finally solidified by Jimmy Carter in 1978.
"[I]t would be a mistake for Beijing and others to over-interpret the meaning of a phone call between President-elect Trump and the President of Taiwan," former Bush White House Asia adviser, Dennis Wilder, said.
He continued: Mr Trump is "not steeped in the diplomatic history of US-China relations and probably has not been briefed by the Department of State on the US-China understandings on our unofficial ties to Taiwan. ...
"We are in uncharted territory with Trump foreign policy, and nations should give him some latitude as he forms his foreign policy team."
Throughout his campaign, Mr Trump had been overtly critical of US policies with China.
"We can't continue to allow China to rape our country," he said at a campaign rally in May.
Mr Trump's call with Ms Tsai comes only hours after China foreign ministry spokesperson's remarks of a new defence spending bill that suggests the US conduct military exchanges with Taiwan.
"China firmly opposes the United States and Taiwan carrying out any form of official contact or military exchange," Mr Geng said, as he urged the US to "scrupulously abide" by decades-old policy or risk damaging relations between the two countries.
