Indian officials say a naval vessel travelling off the coast of Vietnam was confronted by a Chinese warship last month in the South China Sea.

An Indian foreign ministry spokesman says the INS Airavat was 83 kilometers off the coast of Vietnam on July 22, when it received a radio transmission from an unidentified Chinese ship, saying it was entering Chinese waters.

Officials say the Airavat continued its journey when it did not see any ship or aircraft impeding its route.

China has become increasingly assertive in the South China Sea. Six governments lay claim to all or part of the South China Sea, including many of its energy-rich territories. China claims sovereignty over the entire sea, based on an early 20th century map.