Spy photographs obtained by Fairfax Media reveal that China is rapidly building artificial islands on reefs in fiercely disputed areas of the South China Sea.

For centuries they were dots in turquoise waters, home only to sea birds. But the latest images show that land reclamation and construction is underway on Kennan and Burgos islands in the heart of the Spratly Islands and on its eastern and western extremes, as China aggressively presses territorial claims in the resource-rich waters.

Burgos Reef, May 2014.

The United States warned last week that China's accelerated land reclamation work had proved "intimidating and worrisome" for other regional countries, including the Philippines and Vietnam.

US Assistant Secretary of State Daniel Russel, the senior US diplomat for East Asia, said that while Beijing rightly points out it is not the only claimant country to have carried out such work "the pace of China's reclamation in the sensitive disputed waters of the South China Sea vastly outstrips what other claimants have done in the past by many orders of magnitude."