UPDATE 1-U.S. diplomat says bid to decide Taiwan's future by non-peaceful means a 'grave concern'
Washington does not have diplomatic ties with Taipei, but is its main arms supplier and strongest international backer.
This month, Taiwan President Tsai Ing-wen vowed to boost national security and said her government would not submit to Chinese suppression.
"Any effort to determine Taiwan's future by other than peaceful means...is of grave concern to the United States," Brent Christensen, the director of the United States' de facto embassy in Taipei, told reporters. He did not elaborate.
China, which has never renounced the use of force to bring Taiwan under its control, has boosted military and diplomatic pressure on Taipei.
The Chinese military has stepped up encirclement drills around Taiwan, which the island has denounced as intimidation, and three former allies, El Salvador, Burkina Faso and the Dominican Republic, switched ties to Beijing this year.
The United States' new $256-million representative office in Taiwan's capital is an "important symbol" of their partnership, Christensen said, adding that Washington would keep backing Taiwan's "substantive role" in the international community.
China has been infuriated by recent U.S. sanctions on its military, among several flashpoints in ties ranging from a bitter trade war and the issue of Taiwan to China's increasingly muscular military posture in the South China Sea.
Last week, the United States sent two warships through the Taiwan Strait in its second such operation this year, despite opposition from China. (Reporting by Jess Macy Yu; Writing by Yimou Lee; Editing by Clarence Fernandez)
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