"China is shocked and confused by Vietnam's argument that its flights threatened civilian air traffic," Chinese Foreign Ministry spokesman, Hong Lei told a media briefing here.
The Civil Aviation Authority of Vietnam said in a statement on Tuesday that the notification from the Chinese Embassy "cannot substitute China's notification to the appropriate air traffic services units of Vietnam in order to ensure the safety and regularity of flight operations".
Due to unreasonable obstruction from the Vietnamese side, China ultimately decided to turn the flights into a state aviation activity and sent civilian planes to complete the flights, he said.
On January 6, China conducted two test flights at its southernmost airport on Yongshu Jiao, which it claims to be developing for humanitarian purposes, including emergency landing and maritime rescue.
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China claims almost the whole of the South China Sea, resulting in overlapping claims with several other Asian nations like Vietnam and the Philippines, Malaysia and Brunei.
They accuse China of illegally reclaiming land in contested areas to create artificial islands with facilities that could potentially be for military use.