"Any irresponsible comments from other countries cannot be accepted," Lieutenant General He Lei said at the Shangri-La Dialogue in Singapore.

It came just hours after US Defense Secretary Jim Mattis told the security summit that China's military build-up and deployment of weapon systems in the contested sea was aimed at intimidating its neighbours.

Beijing has deployed a range of military hardware including anti-ship missiles, surface-to-air missiles and electronic jammers across the South China Sea, where it has built islets and other maritime features into hardened military facilities.

China has also landed heavy bombers on Woody Island in the Paracel Islands.

The Chinese general, however, said Beijing's actions were aimed at "national defence".

"They are for the purpose of avoiding being invaded by others... As long as it is on your own territory you can deploy the army and you can deploy weapons," he said.

China claims most of the resource-rich sea, through which $5 trillion in shipping trade passes annually, with competing claims from Brunei, Malaysia, the Philippines, Taiwan and Vietnam.