BEIJING—Newly released satellite images show a dramatic expansion in China’s construction of artificial islands on disputed South China Sea reefs, intensifying concerns about Beijing’s territorial ambitions.

The images provide the first visual evidence that China has built an artificial island covering 75,000 square yards—about 14 football fields—and including two piers, a cement plant and a helipad, at a land formation called Hughes Reef, according to experts who have studied the pictures. The reef, which is above water only...