WASHINGTON — The clusters of Chinese vessels busily dredge white sand and pump it onto partly submerged coral, aptly named Mischief Reef, transforming it into an island.

Over a matter of weeks, satellite photographs show the island growing bigger, its few shacks on stilts replaced by buildings. What appears to be an amphibious warship, capable of holding 500 to 800 troops, patrols the reef’s southern opening.

China has long asserted ownership of the archipelago in the South China Sea known as the Spratly Islands, also claimed by at least three other countries, including the Philippines, an American ally. But the series of detailed photographs of Mischief Reef shows the remarkable speed, scale and ambition of China’s effort to literally gain ground in the dispute.

The photographs show that since January, China has been dredging enormous amounts of sand from around the reef and using it to build up land mass — what military analysts at the Pentagon are calling “facts on the water” — hundreds of miles from the Chinese mainland.