The American Institute in Taiwan has functioned as Washington's de facto embassy in democratic self-ruled island Taiwan since 1979.
It was opened to conduct relations with Taiwan following Washington's decision to switch diplomatic recognition to Beijing.
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The new building is a significant upgrade from the low-key military building that AIT has used for decades and will serve as the representative office later this summer, AIT Director Kin Moy said at the opening ceremony.
The site spreads over 6.5 hectares (14 acres), including Chinese gardens, in Taipei's Neihu district. AIT's Taipei office has almost 500 American and local employees, while its Kaohsiung branch has more than 30 staff members.
China's Foreign Ministry has lodged stern representations with the United States about the new AIT building, spokesman Geng Shuang said on Tuesday.
'Close cooperation'Moy said the new building was "a symbol of the close cooperation and enduring friendship between the United States and Taiwan."
The US has no formal diplomatic relations with Taiwan but continues to have close economic, political and security ties with the democratic self-ruled island. It is Taiwan's strongest ally and only foreign arms supplier.
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Taiwanese President Tsai Ing-wen said the complex was a confirmation of the countries' commitment to a "vital relationship."
"The friendship between Taiwan and the US has never been more promising. The great story of Taiwan-US relations remains to be filled with the efforts of those that will one day occupy this building," Tsai said.
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Marie Royce, the US's assistant secretary of state for educational and cultural affairs, said the complex was a symbol of the strength and vibrancy of the countries' partnership.
"We have faced many trials along this journey, but we have risen to the challenge at every turn, knowing that our shared commitment to democracy would see us through," said Royce, the highest-ranking State Department official to visit Taiwan since 2015.
'US cannot be their savior'In an editorial on Tuesday, the state-run newspaper the Global Times called on Chinese officials to warn their counterparts in Taiwan and the United States against provocation.
"The mainland must continue to build up its deterrence against Taiwanese authorities, making them know that the US cannot be their savior," the editorial read.
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China continues to claim the island under its "one China" policy, which Taiwan rejects, and hostility toward Taiwan has only grown in Beijing since President Tsai was elected in 2016.
Officials suspect that Tsai wants to push for formal independence.
Recently, Taiwan lost two diplomatic allies after they switched ties to China, while some international companies have changed their websites to show the island as part of China.
law/rc (AP, Reuters)