China said on Tuesday it had lodged a protest with the United States over President Barack Obama's signing of legislation expressing U.S. support for Taiwan's campaign to attend meetings of a U.N. civil aviation body.
The bill supporting Taiwan's bid for observer status with the International Civil Aviation Organization (ICAO), a U.N. agency that promotes safe air travel, easily passed both chambers of the U.S. Congress in June.
China claims Taiwan as a wayward province though the two sides have been governed separately since defeated Nationalist forces fled to Taiwan in 1949 after losing the Chinese civil war.
China says it alone has the right to represent Taiwan internationally, like at the United Nations, though it has made exceptions for membership of some bodies, such as the World Trade Organization, as long as the island is identified as "Chinese Taipei".
"The joining of international organisations like ICAO by Taiwan compatriots is a matter for the Chinese people," China's Foreign Ministry spokeswoman Hua Chunying said in a statement.
"China opposes any foreign government, group or individual poking their nose in," she added.
The actions by the United States run contrary to the "one-China" principle and China has "lodged stern representations with the U.S. side", Hua said.
"We urge the United States to strictly abide by its promises to the Chinese side on the Taiwan issue and cautiously and appropriately handle matters related to Taiwan and stop interfering in China's internal affairs."
Most countries, including the United States, follow the "one-China" policy of officially recognising Beijing and shunning formal ties with Taiwan.
Beijing and Washington have long argued about democratic and self-ruled Taiwan, most often over U.S. arms sales to the island which enrage China.
Relations between China and Taiwan, however, have improved markedly since 2008 with the election on the island of a China-friendly president, Ma Ying-jeou, and the two sides have signed landmark trade and economic deals.
Taiwan's bid for observer status at the ICAO is expected to be decided when the agency holds its assembly in Montreal in September.
China has not explicitly opposed Taiwan's participation attempt, in keeping with efforts to ensure current good relations remain on track.
In 2009, Taiwan gained observer status at the World Health Assembly, the policy-making body for the U.N. World Health Organization.