U.S. Secretary of State Antony Blinken on Tuesday urged member states of the United Nations to support Taiwan's "robust" participation in U.N. agencies.
The appeal is a direct challenge to China, which claims that Taiwan is part of its territory and that it has sole right to represent the island internationally.
Taiwan held the Chinese seat at the U.N. for more than two decades after the Nationalist government of Chiang Kai-shek retreated to the island as Communist forces overran the mainland in 1949.
The seat was transferred to Beijing in 1971 by a vote of the General Assembly, but Taiwan has since been permitted to participate in some U.N. agencies.
"The fact that Taiwan participated robustly in certain U.N. specialized agencies for the vast majority of the past 50 years is evidence of the value the international community places in Taiwan's contributions," Blinken said in a statement. "Recently, however, Taiwan has not been permitted to contribute to U.N. efforts."
The top U.S. diplomat said Taiwan's exclusion from U.N. forums "undermines the important work of the U.N. and its related bodies, all of which stand to benefit greatly from its contributions."
"That is why we encourage all U.N. member states to join us in supporting Taiwan's robust, meaningful participation throughout the U.N. system and in the international community," he said.
China has said it is "gravely concerned" about U.S. advocacy for Taiwan's participation in global organizations and that Washington's efforts will fail.
In recent months, tensions have increased between Beijing and Taipei, with Beijing sending dozens of warplanes on flights through Taiwan's air defense zone.
Even as it supports a one-China policy, the U.S. sends military aid to Taiwan.
Last week, at a CNN town hall, U.S. President Joe Biden said the United States has a commitment to defend the island and would come to Taiwan's defense if China attacked it.
Later, the White House said there has been no change in U.S. policy.