U.S. stealth jets and nuclear-capable bombers were flown to South Korea on Tuesday hours after Kim Jong Un’s sister threatened to conduct a full-range intercontinental ballistic missile (ICBM) test. The B-52 bombers and F-22 stealth fighters took part in joint drills with South Korean aircraft in the waters southwest of Jeju island in the Korea Strait, Seoul’s defense ministry said. The exercise came after the North Korean dictator’s sister, Kim Yo Jong, commented on doubts about the capabilities of Pyongyang’s nuclear missiles. So far, North Korea’s ICBM tests have been launched at a steep angle to avoid flying into foreign countries’ airspace. Some international observers say the Hermit Kingdom’s ICBMs can’t be considered reliable without being tested in a standard-trajectory launch—though such a test would be a massive provocation to the U.S. as the missile would be flying toward the Pacific Ocean. “I can clear up their doubt about it,” Kim Yo Jong said in remarks covered by state media Tuesday. “They will immediately recognize it in case we launch an ICBM in the way of real angle firing straight off.”