US Defence Secretary Jim Mattis has blasted China for "intimidation and coercion" in the South China Sea and strongly endorsed the "Quad" security grouping of the US, Australia, India and Japan as a way to bolster peace and openness in the Indo-Pacific.

Speaking at an international security forum in Singapore, Mr Mattis rebuked China's military build-up in the disputed waters, warning of increasingly severe consequences should the rising Asian power fail to work collaboratively with neighbouring countries.

The retired general told the Shangri-La dialogue the recent decision to eject China from the Rim of the Pacific multilateral naval exercise was an initial and "relatively small" consequence of Beijing's escalating activities in the South China Sea, which he said were a breach of a 2015 commitment by Chinese President Xi Jinping not to militarise island features in the territory.

"China's militarisation of artificial features in the South China Sea includes the deployment of anti-ship missiles, surface-to-air missiles, electronic jammers and, more recently, the landing of bomber aircraft at Woody Island," Mr Mattis said in his address.