Seoul said the complaint had been lodged with the defence attaches of both countries in the South Korean capital.

"Our military will continue to respond actively to the activities of neighbouring countries' aircraft within the KADIZ in compliance with international law," said Lee Kwang-suk, director general of the International Policy Bureau at Seoul's defence ministry, referring to the Korea Air Defence Identification Zone.

South Korea said Tuesday it had deployed "fighter jets to take tactical measures in preparation for any contingencies" in response to the Chinese and Russia incursion into the KADIZ.

The planes were spotted before they entered the air defence identification zone, defined as a broader area in which countries police aircraft for security reasons but which does not constitute their airspace.

Beijing later in the day confirmed it had organised drills with Russia's military according to "annual cooperation plans".

And Moscow also described it as a routine exercise, saying it lasted eight hours and that some foreign fighter jets followed the Russian and Chinese aircraft.

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

In November last year, Seoul scrambled jets as five Chinese and six Russian military planes flew through its air defence zone.

Similar incidents occurred in June and December 2023, and in May and November 2022.