The fuel in solid-propellant rockets is already loaded inside, which helps to shorten launch preparation times, increase the weapon's mobility and make it harder for outsiders to detect what's happening before liftoff. North Korea already has a growing arsenal of short-range, solid-fueled ballistic missiles targeting key locations in South Korea, including U.S. military bases there. The exact status of North Korea's nuclear attack capability remains in secrecy, as all its intercontinental ballistic missile tests in recent years have been carried out at a steep angle to avoid neighboring countries. Some experts speculate North Korea already has functioning nuclear-tipped missiles that can hit the entire US, given the number of years it has spent on its nuclear program. But others say country is still years away from acquiring such weapons, saying it has yet to publicly prove it has a technology to protect warheads from the harsh conditions of atmospheric reentry.