"China has indisputable sovereignty over the Nansha Islands and their adjacent waters. China's test flights to the newly-built airport on Yongshu Jiao fall totally within China's sovereignty," foreign ministry spokesperson Hong Lei told a media briefing here.
China and Vietnam have counter claims over the Spratly islands which China calls as Nansha islands.
China on Wednesday conducted two test flights landing on and taking off from its southernmost airport on Yongshu Jiao, which it claims to be developing for humanitarian purposes, including emergency landings and maritime rescue.
China claims sovereignty over almost all of the sea.
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Vietnam, the Philippines, Malaysia, Brunei and Taiwan have counter claims on the sea.
"The accusations that China's test flights 'threaten the safety of all flights in the region' are entirely groundless," Hong said in response to reports that Civil Aviation Authority of Vietnam (CAAV) director Lai Xuan Thanh sent a protest letter about the flights to Beijing, as well as a complaint to the UN's International Civil Aviation Organization (ICAO).
"Vietnam's claims that it has not received notification from China does not conform to the facts at all," Hong said.
In addition to notifications via the professional channel, China also gave special technical notification and explanations to Vietnam's foreign ministry on December 30, 2015, Hong added.
"Unfortunately, Vietnam ignored that the test flights are professional, civil and for the international public good, and has continued to disturb China's normal activities for no reason," Hong said.
According to international laws, there is no restriction on national aviation activities in the Convention on International Civil Aviation and other related rules of the ICAO, he added.
Hong said China successfully completed the test flights at the airport on Yongshu Jiao on its own.
The inspection results show that the new airport accords with the standards of civil aviation and has the ability to ensure civilian airplanes' safety, Hong said, adding that it can facilitate personnel exchanges in the region and lift public service capabilities, including maritime rescues in the South China Sea.