A Chinese Xi'an Y-20 Kunpeng military transport aircraft flies during the 15th China International Aviation and Aerospace Exhibition in Zhuhai, in Guangdong Province, Nov. 12. AFP- Yonhap

Five Chinese and six Russian military aircraft entered South Korea's air defense identification zone (KADIZ) over the seas off the eastern and southern coasts without notice on Friday, prompting the South to send its fighter jets in response, the South Korean military said.

Between 9:35 a.m. and 1:53 p.m., the Chinese and Russian military planes entered the KADIZ consecutively over the East Sea and the waters off the southern coast, according to an official at the Joint Chiefs of Staff (JCS).

The Chinese planes reportedly entered the KADIZ near Ieodo Island off the southern coast and flew northward over the waters between South Korea and Japan, while the Russian aircraft approached from the northeast.

They met and flew side by side over waters south of the easternmost islets of Dokdo before retreating.

The aircraft, reportedly including bombers and fighter jets, did not violate South Korea's air space, officials said.

The air defense zone is not territorial airspace but is delineated to call on foreign planes to identify themselves so as to prevent accidental clashes.

The South Korean military detected the planes before they entered the KADIZ and took "tactical" measures by mobilizing Air Force fighter jets in preparation for a potential emergency, the JCS said.

The JCS assessed that the flight was part of a joint air exercise between China and Russia.

Seoul lodged a "stern" complaint against the move, expressing regret and calling for efforts to prevent such actions going forward in separate phone calls with both sides, the defense ministry said.

"The defense ministry strongly requested for an appropriate measure to prevent the recurrence of such actions, which can unnecessarily cause regional tension," the ministry said in a release.

Since 2019, the two countries have sent their military planes into the KADIZ once or twice a year during joint exercises, without prior notice.

Military planes from both China and Russia last entered the KADIZ together in December last year, when they said they were conducting a joint air patrol. (Yonhap)