Taiwan's outgoing president visited an island in the disputed South China Sea on Thursday and called for peaceful development in the increasingly tense region.
The move was likely to infuriate China and other nations that stake a claim to the area, and drew a rare rebuke from staunch ally the United States.
Accompanied by about 30 staff members, Ma Ying-jeou left the capital Taipei aboard an air force C-130 cargo plane bound for Taiping Island, also known as Itu Aba.
Taiwan is spending more than $100m to upgrade the island's airstrip and build a wharf capable of allowing its 3,000-tonne coast guard cutters to dock.
"All this evidence fully demonstrates that Taiping Island is able to sustain human habitation and an economic life of its own. Taiping Island is categorically not a rock, but an island," Ma said.
Taiping lies in the Spratly island group, an area where Taiwan shares overlapping claims with China, Vietnam, Malaysia and the Philippines. The city state of Brunei also claims a part of the South China Sea.
Roughly 2,000km south of Taiwan, Taiping is the largest naturally occurring island in the area.
There was no immediate response to Ma's visit from China, although a spokesman for the Beijing's Taiwan Affairs Office on Wednesday repeated its claim to "indisputable sovereignty" over the South China Sea islands.
The Philippines, which occupies a string of islands and reefs near the island Ma visited, expressed its concern.
"We remind all parties concerned of our shared responsibility to refrain from actions that can increase tension in the South China Sea," Department of Foreign Affairs spokesman Charles Jose said in Manila.
Ma, who has been criticised at home as weak on foreign policy, must step down in May because of term limits after eight years in office, and analysts said he considers the island visit a capstone to his time in office.
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Opposition party president-elect Tsai Ing-wen declined an invitation to go on the trip.
"President Ma ... views advancing [Taiwan's] maritime interests as part of his legacy," said Bonnie Glaser, from the Center for Strategic and International Studies, a think-tank in Washington.
"His visit to Taiping will further incite nationalistic fervour in the claimant countries and increase tensions."
State Department spokesman Mark Toner said the US was disappointed by Ma's trip. "We view it, frankly, as raising tensions rather than what we want to see, which is de-escalation," he said.
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