SEOUL, South Korea (AP) — North Korea flew 12 warplanes near its border with South Korea on Thursday, prompting the South to scramble 30 military aircraft in response, Seoul officials said. The highly unusual incident came hours after North Korea fired two ballistic missiles into the sea in its sixth round of missile tests in less than two weeks.

Eight North Korean fighter jets and four bombers flew in formation and were believed to have conducted air-to-surface firing drills, South Korea's military said.

The military said South Korea responded by scrambling 30 fighter jets and other warplanes, though they didn't engage in any clash with the North Korean aircraft.

The North Korean planes were probably dozens of kilometers away from the border, South Korean media said.

North Korea has previously sent military aircraft near the border, but Yonhap news agency said this is likely the first time it has mobilized so many warplanes for such a provocative flight and firing exercises.