A US B-1B bomber on Wednesday joined large-scale US-South Korean military exercises that North Korea has denounced as pushing the peninsula to the brink of nuclear war, as tension mounts between the North and the United States. Meanwhile, a North Korean ballistic missile was seen by the crew of several airliners last week, aviation authorities said on Wednesday, raising concerns about the threat to civilian flights posed by Pyongyang’s unannounced tests. What started as a high-altitude, high-speed nuclear-penetration bomber, has evolved into today’s all-purpose non-nuclear attack aircraft, a jack-of-all-trades with a huge carrying capacity for bombs and air-to-ground missiles.