China is building a massive island in the South China Sea that could host an airfield in an area where Beijing is locked in bitter territorial disputes with neighbouring states.

US military spokesman Lieutenant Colonel Jeffrey Pool says the vast land reclamation project on the Fiery Cross Reef in the Spratly Islands is one of several pursued by China but the first that could accommodate an airstrip.

A harbour also has been dug out on the east side of the reef that appears large enough for tankers and naval warships.

The United States wants China to halt the project.

In the past three months, China has used dredgers to construct an island about 3000 metres long and 200-300 metres wide on the reef, which was previously under water.