China has dismissed a protest by Vietnam after a Chinese civilian aircraft landed on one of the Nansha Islands, saying that the operation was carried out within Chinese sovereignty.
China carried out a test flight to a newly built airfield on the island of Yongshu Jiao to see whether the facility met civil aviation standards, Foreign Ministry spokeswoman Hua Chunying said on Saturday.
Hua made the remarks in response to objections voiced by Vietnam's Foreign Ministry.
China has "indisputable sovereignty" over the Nansha Islands, and will not accept Vietnam's "unfounded accusations", Hua said.
Relations between the two countries are "maintaining development momentum" and China hopes Vietnam can work to achieve "sustainable, healthy and stable" development of bilateral ties, Hua said.
In 2014, Yongshu Jiao became the largest of the Nansha Islands, with an area of almost 1 square kilometer following reclamation work by China on a coral reef, according to Chinese media reports.
China established a maritime observatory on Yongshu Jiao in the 1980s to collect hydrological and meteorological data in the region that had been commissioned by the United Nations Educational, Scientific and Cultural Organization.
Ruan Zongze, vice-president of the China Institute of International Studies, said, "The airfield on the island will help China to collect more data for such research and to perform better in rescue missions."
Ruan added that countries, including Vietnam, have been "illegally occupying" China's islands and reefs in the Nansha Islands and carrying out construction work on them.
Han Feng, deputy head of the National Institute of International Strategy under the Chinese Academy of Social Sciences, said China had refrained from carrying out major construction work on the islands.
This was because of Beijing's "consistent stance of maintaining regional security". As a result, China lags behind in such construction work compared with other claimants to the South China Sea.
Zhou Fangyin, an expert of China's foreign policies at the Guangdong Institute for International Strategies, said China doesn't aim to militarize the islands.
But "repeated provocations" by the United States, such as sending vessels to the South China Sea, have forced China to ensure it has installations to protect people and facilities on the islands.