By Hyonhee Shin SEOUL (Reuters) – North Korea said on Monday it had tested a new solid-fuel hypersonic missile with intermediate range, amid an intensifying race for the next generation of long-range rockets that are difficult to detect and intercept. The United States, China, Russia and other countries have also been developing hypersonic weapons in recent years. HOW THE MISSILES WORK Hypersonic missiles typically launch a warhead that travels at more than five times the speed of sound or about 6,200 km per hour (3,850 mph), often manoeuvring at relatively low altitudes. Despite their name, a…