Vietnam's civil aviation authority has accused Beijing of threatening regional air safety by conducting unannounced flights through its airspace to a disputed reef in the South China Sea, state media said Saturday.

The Civil Aviation Authority of Vietnam (CAAV) warned that the unannounced flights "threaten the safety of all flights in the region," according to a report in the Tuoi Tre Daily newspaper.

In quotes published in Vietnamese official online newspaper Zing.vn late Friday, CAAV director Lai Xuan Thanh said a protest letter about the flights had been sent to Beijing, as well as a complaint to the United Nations' 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 added.

In the seven days to January 8, Vietnam logged 46 incidents of Chinese planes flying without warning through airspace monitored by air traffic control in the southern metropolis of Ho Chi Minh City, according to civilian aviation authorities quoted in the Tuoi Tre Daily newspaper report.

Chinese state media on Wednesday said two civilian planes landed on an island in the Fiery Cross reef in the contested Spratly Islands, which have long been at the centre of bitter wrangling between Vietnam and its giant neighbour.

The two "test flights" Wednesday followed an initial aircraft landing on Saturday, which prompted the first formal diplomatic complaint from Hanoi.

The Spratlys are claimed by Hanoi but controlled by Beijing, which has ramped up activity in the area by rapidly building artificial islands, including airstrips said to be capable of hosting military jets.

The recent flights, slammed by Vietnam as a "serious violation" of its sovereignty, have sparked international alarm, with the United States warning Thursday that the move would raise tensions in the disputed waters.

The Philippines has also said it would file a protest.

China asserts ownership over virtually all the South China Sea, putting it at odds with regional neighbours the Philippines, Vietnam, Malaysia, Brunei and Taiwan, which stake partial claims.

Several of these nations, including Vietnam, have also built facilities on islands they control, but at a significantly slower pace and smaller scale than Beijing.

Rioting broke out in Vietnam after Beijing sent an oil rig into contested waters in 2014, and at least three Chinese people were killed.

Since then the two sides have tried to mend relations. China's President Xi Jinping visited Hanoi in November but that visit also saw anti-Chinese protests.

Vietnamese officials said last week they had asked Beijing to investigate the ramming and sinking of a Vietnamese fishing boat by a suspected Chinese boat.

Hanoi has stepped up cooperation with the US, in what analysts say is a hedge against China's rising power.