North Korea on Monday fired three ballistic missiles off of its east coast just after noon local time. South Korean officials confirmed the event, as the international G20 summit was being held in China. South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe met at the summit to discuss the situation shortly after learning of the missile launch. The medium-range missiles flew about 600 miles and hit the sea in Japan's air defense zone. "We are still analyzing details, but this is a grave threat to our nation's security, and we express deep concern," the Japan Defense Ministry said in a statement.