US President-elect Donald Trump spoke with Taiwan’s President on Friday, a move that threatens to ignite a diplomatic showdown with China even before he takes office in January.

China has lodged a diplomatic protest and its foreign minister said he hoped Beijing’s relations with the US would not be damaged after Mr Trump broke with decades-long diplomatic tradition and spoke for 10 minutes on the phone with Taiwan’s president Tsai Ing-wen.

It is highly unusual, probably unprecedented, for a US president or president-elect to speak directly with a leader of Taiwan, a self-governing island, since US President Jimmy Carter severed diplomatic ties in 1979.

China’s Foreign Ministry said on Saturday it had lodged “stern representations” with what it called the “relevant US side”, urging careful handling of the issue to avoid unnecessary disturbances in ties.

The wording implied the protest had gone to the Trump camp but the ministry provided no explanation.

Chinese Foreign Minister Wang Yi said the call between Dr Tsai and Mr Trump was “just a small trick by Taiwan” that he believed would not change US policy toward China, according to Hong Kong’s Phoenix TV.

“The one-China policy is the cornerstone of the healthy development of China-US relations and we hope this political foundation will not be interfered with or damaged,” Wang was quoted as saying.

Washington has pursued a so-called “one China” policy shifting diplomatic recognition of China from the government in Taiwan to the communist government on the mainland.

Under that policy, the US recognises Beijing as representing China but retains unofficial ties with Taiwan.
 Despite the lack of formal diplomatic ties, the US remains Taiwan’s most important political ally and sole arms supplier.

But the phone call comes at a time of worsened Taiwan-China relations since the election of Dr Tsai’s pro-independence Democratic Progressive Party (DPP) earlier this year.

A statement from Trump’s transition team said he spoke with Dr Tsai, who offered her congratulations.

“During the discussion, they noted the close economic, political, and security ties … between Taiwan and the United States. President-elect Trump also congratulated President Tsai on becoming President of Taiwan earlier this year,” the statement said.

Trump tweeted later that Dr Tsai had initiated the contacted with him.

The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you!

— Donald J. Trump (@realDonaldTrump) December 3, 2016

Interesting how the U.S. sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call.

— Donald J. Trump (@realDonaldTrump) December 3, 2016

The Taiwanese presidential office issued a statement early Saturday saying Trump and Tsai discussed issues affecting Asia and the future of US relations with Taiwan.

“The (Taiwanese) president is looking forward to strengthening bilateral interactions and contacts as well as setting up closer co-operative relations,” the statement said.

“The president also told US President-elect Trump that she hopes the US will continue to support Taiwan’s efforts in having more opportunities to participate in and contribute to international affairs in the future,” Dr Tsai’s office said.

Obama breaks w/decades of US policy on Cuba & gets endless fawning coverage. Trump breaks w/US policy by phoning Taiwan & he's reckless?

— Stephen Hayes (@stephenfhayes) December 3, 2016

During his election campaign Trump took a a tough line towards Beijing, branding it a currency manipulator and warning that it was committing “rape” against American workers with its trade policy.

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