NEW YORK (AP) — President-elect Donald Trump spoke Friday with the president of Taiwan, a move that will be sure to anger China.

It is highly unusual, perhaps unprecedented, for a U.S. president or president-elect to speak directly with a leader of Taiwan, a self-governing island the U.S. broke diplomatic ties with in 1979.

Washington has pursued a so-called "one China" policy since 1979, when it shifted diplomatic recognition of China from the government in Taiwan to the communist government on the mainland. Under that policy, the U.S. recognizes Beijing as representing China but retains unofficial ties with Taiwan.

A statement from Trump's transition team said he spoke with Taiwanese President Tsai Ing-wen, who offered her congratulations. It was not clear who initiated the call.

"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," the statement said.

A Taiwanese source with direct knowledge of the call confirmed it had taken place. The source requested anonymity to speak about it before an official statement was issued on it from Taipei.

China's embassy in Washington did not immediately respond to a request for comment.

Friday's call is the most stark example yet of how Trump has flouted diplomatic conventions since he won the Nov. 8 election. He has apparently undertaken calls with foreign leaders without guidance customarily lent by the State Department, which oversees U.S. diplomacy.

Over the decades, the status of Taiwan has been one of the most sensitive issues in U.S.-China relations. China regards Taiwan as part of its territory to be retaken by force, if necessary, if it seeks independence. It would regard any recognition of a Taiwanese leader as a head of state as unacceptable.

Taiwan split from the Chinese mainland amid civil war in 1949. The U.S. policy acknowledges the Chinese view over sovereignty, but considers Taiwan's status as unsettled.

Although the U.S. does not have formal diplomatic ties with Taiwan, it has close unofficial ties. Taiwan's government has a representative office in Washington and other U.S. cities. The U.S. also has legal commitments to help Taiwan maintain the ability to defend itself.

Tsai was democratically elected in January and took office in May. The traditional independence-leaning policies of her party have strained relations with Beijing.

Diplomatic protocol dictates that Taiwanese presidents can transit through the U.S. but not visit Washington.