Five Chinese and six Russian military aircraft entered and exited the Korea Air Defence Identification Zone in the East Sea and South Sea from 9:35 am (0035 GMT) to 1:53 pm, Seoul's Joint Chiefs of Staff said in a statement.

But the planes crossed into the KADIZ "without violating South Korean airspace", the JCS said, adding that the military "identified the aircraft before they entered KADIZ and deployed Air Force fighter jets to take tactical measures in preparation for any contingencies".

An air defence identification zone is a broader area than a country's airspace in which it tries to control aircraft for security reasons, but the concept is not defined in any international treaty.

China's defence ministry called the flights "their ninth joint strategic patrol", which occurred above the Sea of Japan, also known as the East Sea in South Korea.

It added in a post on its official social media account that the flights were held Friday "according to the annual cooperation plan between the Chinese and Russian militaries".

South Korean military officials told local media that Chinese military aircraft flew towards the small Dokdo islands off South Korea's eastern coast, after passing between the Korean peninsula and Japan near the contested submerged rock of Ieodo.

Russian planes also flew south towards the Dokdo islets, known as Takeshima in Japanese.

Officials said the Chinese and Russian aircraft flew together over the sea south of Dokdo before departing.

Since 2019, China and Russia have regularly flown military aircraft into South Korea's air defence zone without prior notice, citing joint exercises.

Similar incidents occurred in June and December last year, and in May and November 2022, with Beijing and Moscow describing the flights as "joint strategic air patrols".

South Korea's defence ministry on Friday "expressed regret" to China and Russia over their military aircraft entering the South's air defence zone and "flying for an extended period without prior notice", according to a statement.

The ministry requested that "appropriate measures be taken to prevent a recurrence", adding that such actions could "unnecessarily heighten tensions in the region".

China and Russia have expanded military and defence ties since Moscow ordered troops into Ukraine nearly three years ago.

Both are also traditional allies of North Korea, Seoul's arch-foe.

South Korea and the United States have accused nuclear-armed North Korea of sending thousands of soldiers to Russia to fight in Ukraine and this month Pyongyang ratified a landmark defence pact with Moscow.