South Korea's defense ministry lodged a "stern" protest with China and Russia on Friday over their warplanes' entry into the country's air defense identification zone (KADIZ) in the East Sea without notice.

On Thursday, two Chinese and four Russian military aircraft entered the KADIZ and exited it, prompting the South Korean Air Force to scramble fighter jets to the scene. They did not violate South Korea's air space.

Lee Seung-bom, director general for international policy at the ministry, expressed regret over the KADIZ entry to military attaches at the Chinese and Russian embassies in Seoul, respectively, in separate phone calls.

"The defense ministry expressed regrets to China and Russia over their military aircraft's flight into the sensitive areas close to our air space without prior notice," the ministry said in a press release.

The ministry also called on them to take appropriate measures to prevent a recurrence, noting that such a flight could cause regional tensions.

The foreign ministry said it also expressed regrets to China and Russia over the incident through diplomatic channels, and urged them to prevent a recurrence.

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

China and Russia's defense ministries said Thursday they staged a joint reconnaissance flight under their annual cooperation plans, claiming the flights are in line with international laws. (Yonhap)