Vietnam "resolutely opposes" Taiwan President Ma Ying-jeou's plan to visit the disputed island of Itu Aba, its most senior official in Taiwan said Wednesday.
Both Taiwan and China claim most of the South China Sea. Vietnam, the Philippines, Malaysia and Brunei have competing claims.
"If he goes it will raise tensions in the area," said Tran Duy Hai, representative of the Vietnam Economic and Cultural Office in Taipei, the de facto embassy for Vietnam in the absence of official diplomatic ties.
Vietnam's opposition to the trip comes after the United States, a major ally of Taiwan, said the trip was "extremely unhelpful" and would not do anything to resolve disputes over the waterway.
Ma's office earlier announced that the president, who steps down in May, would fly to Itu Aba on Thursday to offer Chinese New Year wishes to residents on the island, mainly Taiwanese coast guard personnel and environmental scholars.