A total of 19 military aircraft from Russia and China entered South Korea's air defense identification zone on Tuesday without prior authorization, prompting the South Korean Air Force to scramble fighter jets.
Fifteen of the foreign planes were Russian and the other four were Chinese.
Some crossed into the zone near the southern island of Ieodo and some near the eastern islands of Ulleung-do and Dokdo.
According to South Korea's Joint Chiefs of Staff, the Chinese side said the flights were part of a joint military exercise by the two countries.
Seoul's defense ministry said it has lodged complaints with the military attaches of China and Russia, urging them to communicate such maneuvers in advance to prevent such incidents from happening in the future.