Hanoi accused Beijing of making "erroneous statements" in the latest round of an escalating diplomatic spat over flights to a disputed reef in the South China Sea.

China has conducted several flights this year through Vietnamese airspace to a newly built runway on the Fiery Cross reef in the Spratly Island group, which is claimed by both countries.

Vietnam has accused Beijing of threatening regional safety with the "unannounced" flights, but China hit back, arguing it has the right to conduct "state aviation activities".

Hanoi fired off the latest barb late Tuesday in the increasingly bitter diplomatic spat, saying China was making "erroneous statements" by claiming it had informed Vietnamese air traffic control about the flights.

"Ho Chi Minh... Control Centre did not receive any notification by China of the flight plans," Foreign Ministry Spokesman Le Hai Binh said in a statement released late Tuesday.

The flights are "aimed at asserting their illegal sovereignty claims over the Trong Sa archipelago," he said, referring to the Spratlys by their Vietnamese name.

Vietnam this year logged at least 46 incidents of Chinese planes flying without warning through airspace monitored by air traffic control in the southern metropolis of Ho Chi Minh City, formerly known as Saigon, according to authorities cited by local media.

State media also reported that Vietnam sent a protest letter about the flights to Beijing, as well as the United Nations' International Civil Aviation Organization (ICAO).

Beijing began work in 2014 on a 3,000-metre (9,800-foot) runway on Fiery Cross reef, around 1,000 kilometres (600 miles) from China's island province of Hainan.

China's claim to almost all of the South China Sea puts it at odds with regional neighbours the Philippines, Vietnam, Malaysia, Brunei and Taiwan, which also stake partial claims.