Taiwan President Ma Ying-jeou's planned trip to the Taiwanese-held island of Itu Aba in the disputed South China Sea is "extremely unhelpful" and won't do anything to resolve disputes over the waterway, a US official says.
Ma's office earlier announced 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 coastguard personnel and environmental scholars.
But Ma's one-day visit to Itu Aba, known as Taiping in Taiwan, comes amid growing international concern over rising tensions in the waterway and quickly drew the ire of the American Institute in Taiwan (AIT), the de facto US embassy in Taipei in the absence of formal diplomatic ties.
"We are disappointed that President Ma Ying-jeou plans to travel to Taiping Island," AIT spokeswoman Sonia Urbom said in an email to Reuters.
"Such an action is extremely unhelpful and does not contribute to the peaceful resolution of disputes in the South China Sea."
The United States wanted Taiwan and all claimants to lower tensions, rather than taking actions that could raise them, Urbom added.
On a visit to Beijing on Wednesday, US Secretary of State John Kerry said Washington and Beijing needed to find a way to ease tensions in the South China Sea, through which $US5 trillion ($A7.13 trillion) in ship-borne trade passes every year.
Both Taiwan and China claim most of the South China Sea. Vietnam, the Philippines, Malaysia and Brunei also have competing claims.
Asked to comment on Ma's planned visit, the mainland's Taiwan Affairs Office reiterated that China and Taiwan had a common duty to protect Chinese sovereignty in the waterway.
"Safeguarding national sovereignty and territorial integrity as well as safeguarding the overall interests of the Chinese nation is the common responsibility and obligation of compatriots across the straits," spokesman Ma Xiaoguang told reporters in Beijing.
The claims of both China and Taiwan are based on maps from the late 1940s belonging to the Nationalists, when they ruled all of China. The Nationalists fled to Taiwan in 1949 after being defeated in a civil war with China's Communists.
Itu Aba was now the fourth largest island in the Spratlys after China's land reclamation work on Mischief Reef, Fiery Cross Reef and Subi Reef, Taiwan's coastguard said in October.
The island supports around 180 people, about 150 of them coastguard personnel who have had oversight of the 46-hectare island since 2000.
