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... While the test was in line with Kim's stated goal of acquiring more agile, hard-to-detect missiles targeting the U.S. and its allies, some experts speculate North Korea's claim may be an exaggeration. Missiles with built-in solid propellants are easier to move and conceal their launches than liquid-fuel weapons, which in general must be fueled before liftoffs and cannot last long.

The official Korean Central News Agency reported Kim watched the ground jet test of the engine using a composite carbon fiber material. It said the engine's maximum thrust is 2,500 kilonewtons, up from about 1,970 kilonewtons reported in a similar solid-fuel engine test in September.

KCNA reported the test was conducted as part of the country's five-year arms build-up meant to upgrade "strategic strike means," a term referring to nuclear-capable ballistic missiles and other weapons. Kim said the latest engine test had "great significance in putting the country's strategic military muscle on the highest level," according to KCNA. The agency did not say when or where the test occurred.

North Korea's report on the latest test could be "bluffing" as it didn't disclose some key information like the engine's total combustion time, said Lee Choon Geun, an honorary research fellow at South Korea's Science and Technology Policy Institute. ...