The United States unveiled a new $256 million representative office in Taiwan's capital on Tuesday, a de facto embassy that underscores Washington's strategic ties with the self-ruled island as it faces escalating tensions with China.

Washington cut diplomatic ties with Taipei in 1979 but remains the island's strongest ally and sole foreign arms supplier. It opened the American Institute of Taiwan (AIT) to conduct relations between the two sides after severing ties.

The recently completed American Institute in Taiwan office in a suburb of the capital Taipei will house U.S. representatives and serves American interests in the absence of formal diplomatic ties. AIT Director Kin W. Moy — the U.S. ambassador in all but name — Taiwan's President Tsai Ing-wen and a senior State Department official from Washington, attended Tuesday's ceremony, which Beijing slammed as harmful to U.S.-China ties.

China's Foreign Ministry said yesterday it had lodged stern representations with the United States about the unveiling of the new de facto U.S. embassy in self-ruled Taiwan. "We urge the U.S. to honor its words on the Taiwan issue, and redress its wrongdoing so as to avoid doing harm to China-US ties and peace and stability in cross-strait relations," Chinese Foreign Ministry spokesman Geng Shuang said as reported by AP.

Taiwan's relations with the United States have grown closer amid Washington's own struggles with Beijing. Trump upended decades of diplomatic protocol by accepting a congratulatory phone call from Tsai shortly after his 2016 election. In March, he signed the Taiwan Travel Act that encourages high-level visits between the two sides. A month later, the Department of Defense agreed to give American contractors marketing licenses for diesel-electric submarine technology sought by Taiwan's armed forces. And last month, the White House accused China of "Orwellian nonsense" for demanding that international airlines refer to Taiwan as a part of China on their websites.