Vietnam has warned China twice in a week against threatening regional air safety by conducting unannounced flights over its airspace to a contested reef in the South China Sea, state media said on Saturday.
Vietnam's Foreign Ministry spokesman Le Hai Binh said in a statement issued on late Thursday that the landings are "a serious violation of Vietnam’s sovereignty and threaten peace and stability in the region."
Foreign officials and analysts believe the landing shows Chinese's facilities in the problematic region are being completed according to plan.
A first aircraft landing on Saturday, which triggered the first formal diplomatic accusation from Hanoi, came after two “test flights” on Wednesday.
Civil Aviation Authority of Vietnam (CAAV) director Lai Xuan Thanh said a protest letter about the move had been sent to China and that Vietnam has made a complaint to the UN International Civil Aviation Organization (ICAO).
"Chinese aircraft have ignored all the rules and norms of the ICAO by not providing any flight plans or maintaining any radio contact with Vietnam's air traffic control centre," he said.
The incident has increased tensions in the region and the US said the flights would raise alarm in the disputed waters. The Philippines has also indicated it would file a protest.
China claims the islands are being built for civilian purposes.
After satellite photographs showing the construction of three military length airstrips that could be used for military purposes were taken, the US, Japan and Philippines stated that they are concerned by China’s expansion of its operation in the area.
China claims the majority of the South China Sea, through which at least $5 trillion in global trade passes each year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan also claim parts of the sea.