Taiwan's President Ma Ying jeou's planned visit to a disputed island on the South China Sea is "extremely unhelpful" and won't do anything to resolve disputes over the islands, a US official said on Wednesday.
Ma's office had earlier announced that the president, who will step down in May, would fly to Itu Aba on Thursday to offer Chinese New Year wishes to personnel and environmental scholars stationed there.
Ma's visit follows elections won by the independence-leaning Democratic Progressive Party (DPP). Ma's office said it had asked DPP leader Tsai Ing wen to send a representative, but the party said it had no plans to do so.
Itu Aba is the biggest island in the Spratlys and is known as Taiping in Taiwan.
"We are disappointed that President Ma Ying jeou plans to travel to Taiping Island," said Sonia Urbom, the spokeswoman of American Institute in Taiwan (AIT), the de facto US embassy in Taipei.
"Such an action is extremely unhelpful and does not contribute to the peaceful resolution of disputes in the South China Sea."
The US wants all claimants to lower the tension in the region, Urbom added.
US Secretary of State John Kerry said Washington and Beijing had to make progress on "concerns and activities in the South China Sea," during his visit to China on Wednesday.
Vietnam also issued a statement on Wednesday saying that it "resolutely opposes" Ma's planned visit to Itu Aba.
Besides Taiwan, a US ally, the archipelago is claimed partly or in whole by China, the Philippines, Vietnam, Malaysia and Brunei.
$5 trillion in ship-borne trade passes every year through the South China Sea.
Beijing still sees Taiwan as a part of its “one China,” although it has been a self ruling state since 1949.
"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.
Yet, Taiwan conducted military drills this week in the face of growing fears that tensions with China will escalate as Tsai Ing wen will replace Ma, who is from China-friendly Kuomintang (KMT).
The DPP is traditionally a pro-independence party and relations with Beijing are likely to cool following a rapprochement under the KMT.
Earlier on Wednesday, Taiwan said it wanted to reaffirm its sovereignty in Taiping.
"Taiping Island is an inherent part of the Republic of China's territory," said Charles Chen, spokesman for Taiwan presidential office.
Taiwan has just finished a $100 million port upgrade and built a new lighthouse on the island, which has its own airstrip, a hospital and fresh water.
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 had fled to Taiwan in 1949 after losing the Chinese civil war to Mao's Communists.