Beijing has rejected a protest from Vietnam after a Chinese plane landed on a contested reef in the South China Sea, saying the operation took place within Chinese territory.

A Chinese "test flight" landed on Fiery Cross reef, foreign ministry spokeswoman Hua Chunying said in an online statement late Saturday. Vietnam also claims the reef.

China has asserted its claim to almost all of the South China Sea by rapidly building artificial islands including airstrips said to be capable of hosting military jets.

It began work in 2014 on a 3,000-metre runway on Fiery Cross reef in the Spratlys island group, around 1,000 kilometres from China's island province of Hainan.

Hua said the test flight was civilian in nature, adding that the "relevant activity falls completely within China's sovereignty".

Hanoi earlier strongly protested at the flight, labelling it a violation of sovereignty which "influences peace and stability in the South China Sea".

"Vietnam resolutely protests China's above-mentioned action, asking China to immediately end while not repeating similar move," said foreign ministry spokesman Le Hai Binh in Hanoi.

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

The fishermen were around 60 nautical miles from Con Co Island in central Quang Tri province on Friday when a foreign boat crashed into their craft.

The 11 crew members were rescued but the boat sank, the fishermen told the VNExpress news site. The captain was quoted as saying that he saw Chinese characters on the foreign boat.

Ha Le, deputy head of the Vietnam Fisheries Surveillance Department, told AFP Chinese officials had offered to check on the report if more details became available.

Relations between the communist neighbours have grown tense in recent years over the disputed Spratly and Paracel island chains.