SEOUL, South Korea (AP) — North Korean leader Kim Jong Un observed a test of an upgraded solid-fuel engine for weapons capable of reaching the U.S. mainland and called it a significant development boosting his country’s strategic military arsenal, state media reported Sunday. 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-