China has installed rocket launchers on a disputed reef in the South China Sea to ward off Vietnamese military combat divers, according to a state-run newspaper, offering new details on China's ongoing military build-up.

China has said military construction on the islands it controls in the South China Sea will be limited to necessary defensive requirements, and that it can do what it likes on its own territory.

The United States has criticised what it has called China's militarisation of its maritime outposts and stressed the need for freedom of navigation by conducting periodic air and naval patrols near them that have angered Beijing.

The state-run Defence Times newspaper, in a Tuesday report on its WeChat account, said Norinco CS/AR-1 55mm anti-frogman rocket launcher defence systems with the capability to discover, identify and attack enemy combat divers had been installed on Fiery Cross Reef in the Spratly Islands.

Fiery Cross Reef is administered by China but also claimed by the Philippines, Vietnam and Taiwan.

The report did not say when the defence system was installed, but said it was part of a response that began in May 2014, when Vietnamese divers installed large numbers of fishing nets in the Paracel Islands.

China has conducted extensive land reclamation work at Fiery Cross Reef, including building an airport, one of several Chinese-controlled features in the South China Sea where China has carried out such work.

More than $5 trillion of world trade is shipped through the South China Sea every year. Besides China's territorial claims in the area, Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.