South Korea’s military said Friday it scrambled fighter jets as five Chinese and six Russian military planes flew through its air defense zone, an area wider than the country’s airspace.

Five Chinese and six Russian military aircraft entered and exited the Korea Air Defense 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 defense 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 defense 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 defense 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 defense ministry on Friday “expressed regret” to China and Russia over their military aircraft entering the South’s air defense 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 defense 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 defense pact with Moscow.

The South Korean jets being scrambled marks the latest incident involving Russian and Chinese militaries in recent months.

Earlier this month, Italy and Norway mobilized jets after Russian aircraft were spotted over the Baltic Sea and along the Norwegian coast. 

In September, Japan said its warplanes used flares to warn a Russian reconnaissance aircraft to leave northern Japanese airspace.

That same month, the U.S. military moved about 130 soldiers along with mobile rocket launchers to a desolate island in the Aleutian chain of western Alaska amid a recent increase in Russian military planes and vessels approaching American territory. Eight Russian military planes and four navy vessels, including two submarines, have come close to Alaska in recent days as Russia and China conducted joint military drills.

In July, two Russian Tu-95s and two Chinese H-6s entered the Alaska Air Defense Identification Zone, NORAD said. The bombers were intercepted by U.S. F-16 and F-35 fighter jets, along with Canadian CF-18s and other support aircraft, a U.S. defense official confirmed to CBS News.