TAIPEI, Taiwan (AP) — Fresh from a landslide reelection victory, Taiwanese President Tsai Ing-wen met Sunday with the de facto U.S. ambassador to Taipei, as China warned that countries should stick with recognizing communist-ruled Beijing as the rightful government of “one China," including Taiwan.

William Brent Christensen, a U.S. diplomat who is director of the American Institute in Taiwan, congratulated Tsai on her victory in Saturday's election, and she thanked him for his support.

China considers self-governed Taiwan a part of its territory and opposes any official contact with the U.S. as an interference in its domestic affairs. The U.S. does not have formal diplomatic relations with Taiwan but is legally bound to ensure that the island can defend itself against threats.

Tsai has sought closer relations with the U.S. while pushing back against pressure from China, and the Trump administration has reciprocated.

Since separating from China during civil war in 1949, Taiwan has developed its own identity but never declared formal independence. Beijing still claims sovereignty over the island of 23 million people and threatens to use force to seize control if necessary.

In her victory speech Saturday night after her rival, Han Kuo-yu of the Nationalist Party, conceded defeat, Tsai urged China to resume talks with Taiwan without preconditions while warning against threatening use of force.

“Today I want to once again remind the Beijing authorities that peace, parity, democracy and dialogue are the keys to stability," Tsai said. “I want the Beijing authorities to know that democratic Taiwan and our democratically elected government will never concede to threats."

Tsai's victory is a setback for Chinese President Xi Jinping at a time when Beijing is grappling with an economic slowdown and long-running, sometimes violent anti-government demonstrations in Hong Kong.

After election results were announced late Saturday, Ma Xiaoguang, a spokesman for China's Taiwan Affairs Office, said China is willing to work with the Taiwanese people to advance the “peaceful reunification of the country."

But he cautioned that China would firmly protect its territorial integrity and opposes any separatist moves and independence for Taiwan, according to China's official Xinhua News Agency.

“We hope and believe that the international community will continue adhering to the One China principle, understand and support the just cause of Chinese people to oppose the secessionist activities for ‘Taiwan independence' and realize national reunification," said a statement from Chinese Foreign Ministry spokesman Geng Shuang.

Opinion polls had showed Tsai leading before the election, but a huge turnout helped her claim more than 8 million votes. She got 57% of the vote to Han's 39%. Her Democratic Progressive Party also managed to win in areas that often go to the China-friendly Nationalists in central and southern Taiwan. It retained its majority in the 113-seat legislature, though its tally of 61 seats was down seven from 2016. The Nationalists won 38 seats, a gain of three.

Tsai has acknowledged that Beijing may well up its pressure on Taiwan following her victory, after cutting off formal ties with her government over the past two years, restricting visits by Chinese tourists and seeking to further isolate Taipei by luring away more of its few diplomatic allies, which now number only 15. Chinese military exercises across the Taiwan Strait and air patrols around the island have added to tensions.

But Tsai received a resounding public mandate for her rejection of China's suggestion for a “one country, two systems" approach to governing Taiwan after months of anti-government protests in Hong Kong, a semi-autonomous Chinese territory, convinced many in Taiwan that such a plan is unworkable.

“We have sent out a crystal-clear message — there are certain values that we are protecting," said one voter, Rocky Hwang, an interior designer. “This is a basic opinion that says Taiwanese people want to decide on their future on their own, and protect our own values.”

Since its transition to full democracy beginning in the 1980s, Taiwan has increasingly asserted its independent identity from China even though it is not recognized by the United Nations or any major nation. Saturday's election was its seventh presidential vote since the island began a transition from martial law under the Nationalists to democracy.

The island exercises all the roles of a sovereign nation, issuing its own passports, maintaining its own military and legal system and serving as an important hub in the global high-tech supply chain.

With the election behind her, Tsai now faces the challenges of keeping economic growth on track and delivering on promises to improve the lives of Taiwan's people.

“It's important to enable younger people to make a better living — this is what we expect," said Lin Li-li, who with her husband, Wang Wen-long, was preparing to return home after visiting Taipei to cast their votes.

The focus now, said Wang, is “to have a more harmonious society and enhance our living.”

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Associated Press videojournalists Johnson Lai and Emily Wang contributed to this report.