Japan's air defense zone extends into South Korea's airspace over its territorial waters near the southernmost island of Marado, the defense ministry spokesman said Tuesday.
The violation happened because Japan set its air defense identification zone in 1969 when territorial waters of a country were considered to be within three nautical miles from its territorial sea baseline, ministry spokesman Kim Min-seok said.
The concept of territorial waters was later extended to 12 nautical miles from the baseline.
"As our territorial waters and airspace were expanded, our aircraft do not notify Japan when operating" over the areas, Kim said.
A country sets an air defense zone to determine whether planes entering the zone are civilian or military.
The issue came to a head after China on Saturday expanded its air defense zone over the East China Sea, obligating all aircraft entering the area to report to Chinese authorities and follow their instructions.
The Chinese zone also overlaps South Korea's and includes airspace over the South Korean-controlled submerged rock of Ieodo.
South Korea has lodged a complaint with China.
The government in Seoul came under fire for failing to include Ieodo in its air defense zone. Defense Minister Kim Kwan-jin said Tuesday his ministry will discuss expanding the zone with other agencies.
China's decision sparked fresh tensions between Beijing and Tokyo over the sovereignty of a set of islands in the East China Sea, which are known as Diaoyu in China and Senkaku in Japan, because the new zone includes airspace over the disputed islands. (Yonhap News)