Taiwan's president, defying a rare criticism from key ally the United States, visited an island in the disputed South China Sea on Thursday to emphasize Taiwan's sovereignty claims in the increasingly tense region.

Accompanied by about 30 staff members, Ma Ying-jeou (MAH' YEENG' JOH') spoke at a national monument on Taiping, also known as Itu Aba, and reiterated his call made last year for peaceful coexistence and joint development with other claimants. The island is part of the Spratly archipelago, where China, Vietnam, Malaysia, the Philippines and the city-state of Brunei have overlapping claims.

Roughly 2,000 kilometers (1,200 miles) south of Taiwan and 46 hectares (110 acres) in size, Taiping is the largest naturally occurring island in the area. It has recently been eclipsed in size, however, by man-made islands created by China out of reefs and shoals. China has built housing, ports, airstrips and other infrastructure on the newly created islands, drawing accusations from the U.S. and others that it is exacerbating tensions in the strategically vital region.

Ma cited infrastructure developments, including a 10-bed hospital and a lighthouse, saying they reinforced Taiwan's claim of sovereignty and granted it rights over the surrounding waters. Taiwan is spending more than $100 million to upgrade the island's airstrip and build a wharf capable of allowing its 3,000-ton 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.

The Philippines expressed concern over the trip, and U.S. State Department spokesman Mark Toner said Wednesday that the United States was disappointed, saying it could exacerbate tensions.

"President Ma Ying-jeou has every right to make his position clear on the South China Sea. We just disagree with this particular action. We view it as, frankly, as raising tensions rather than what we want to see, which is de-escalation," Toner said.

During a visit to Beijing on Wednesday, U.S. Secretary of State John Kerry encouraged all parties in the South China Sea to clarify their territorial claims, exercise restraint and engage in negotiations on the basis of international law.

Taiwan stations about 200 coast guard personnel, scientists and medical workers on Taiping. It occupies a number of other islets in the South China Sea, including the Pratas island group to the north.

There was no immediate response to Ma's visit from China, although a spokesman for the Cabinet's Taiwan Affairs Office on Wednesday repeated Beijing's claim to "indisputable sovereignty" over the South China Sea islands.

"Safeguarding national sovereignty and territorial integrity and the overall interests of the Chinese nation are the common responsibility and obligation of compatriots on both sides" of the Taiwan Strait, Ma Xiaoguang told reporters.

China and Taiwan hold identical claims to the South China Sea, aligning with Beijing's "one China principle" that considers the two part of a single Chinese nation. Beijing has threatened to retaliate to any formal change in Taiwan's legal status with military force.

Coming near the end of his eight years in office, Ma's visit is the second by a Taiwanese leader. Former president Chen Shui-bian visited in 2008 when he delivered a similar message.

Ma, who has been criticized at home as weak on foreign policy, must step down in May due to term limits and analysts said he considers the island visit a capstone to his time in office. Opposition party president-elect Tsai Ing-wen declined an invitation to go on the trip.

Tsai won a decisive victory over the candidate from Ma's China-friendly Nationalist Party in this month's election while leading her independence-leaning Democratic Progressive Party to a majority in the legislature, casting new uncertainty over the future of Taiwan-China relations.

"President Ma...views advancing (Taiwan's) maritime interests as part of his legacy," said Bonnie Glaser, senior adviser for Asia at the Center for Strategic and International Studies, a think tank in Washington. "His visit to Taiping will further incite nationalistic fervor in the claimant countries and increase tensions."


Bodeen reported from Beijing. Associated Press writer Jim Gomez in Manila, Philippines, contributed to this report.