The back-to-back launches, the North’s first ballistic missile tests since Oct. 14, came on the final day of South Korea’s annual 12-day “Hoguk” field exercises, which also involved an unspecified number of U.S. troops this year.North Korea sees such regular drills by Seoul and Washington as practice for launching an attack on the North, though the allies say their exercises are defensive in nature.North, South Korea fire off dueling missile-launch videosUN nuclear chief: North Korea nuke test would be key concernUS, allies warn decisive response if North Korea tests nukeUS vows full military defense of allies against North KoreaNext week’s “Vigilant Storm” aerial drills are to run from Monday to Friday and involve about 140 South Korean warplanes and about 100 U.S. aircraft.Rafael Grossi, chief of the International Atomic Energy Agency, said Thursday that a new nuclear test explosion by North Korea “would be yet another confirmation of a program which is moving full steam ahead in a way that is incredibly concerning.”He said the U.N. agency has been observing preparations for a new test, which would be the North’s seventh overall, but gave no indication of whether an atomic blast is imminent.On Monday, the rival Koreas exchanged warning shots along their disputed western sea boundary, a scene of past bloodshed and naval battles, as they accused each other of violating the boundary."