When a U.S. Navy guided-missile destroyer sailed near one of Beijing's artificial islands in the disputed South China Sea last week, it was operating in a maritime domain bristling with Chinese ships.

While the U.S. Navy is expected to keep its technological edge in Asia for decades, China's potential trump card is sheer weight of numbers, with dozens of naval and coast guard vessels routinely deployed in the South China Sea.

Asian and U.S. naval officers say encounters with Chinese vessels, once relatively rare, are now frequent, even at the outer edges of the controversial "nine-dash line" Beijing uses to stake its claim to 90 percent of the waterway.