A new report to the U.S. Congress assessing a Chinese submarine-launched missile known as the YJ-18 highlights the danger, noting the missile accelerates to supersonic speed just before hitting its target, making it harder for a crew to defend their ship.

Defense chiefs from several countries in Southeast Asia have warned in recent months of the danger of undersea "clutter" as countries build up submarine fleets and the U.S. challenges China over its claim to a large swath of the South China Sea. This week's U.S. patrol inside the 12-nautical mile zone that China claims around its man-made islands in the waters saw the USS Lassen shadowed by two Chinese naval vessels.