The Chinese and Russian planes entered and exited the Korea Air Defence Identification Zone (KADIZ) in the East Sea, also known as the Sea of Japan, from 11:53 am (0253 GMT) to 12:10 pm, Seoul's Joint Chiefs of Staff said.

But "there was no invasion of airspace", it added, and the South Korean military identified the planes "before they entered KADIZ, and deployed air force fighter jets to take tactical measures in preparation for 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 foreign ministry spokeswoman Mao Ning described the incident on Thursday as "a routine flight activity".

"As far as I know, I think this was a routine flight activity by Chinese military aircraft above international waters, which is understandable and in accordance with international law," she told a regular press briefing.

China and Russia are North Korea's traditional allies, and Washington warned last month that military ties between Pyongyang and Moscow were "growing and dangerous".

The United States has called on Beijing -- the North's biggest economic benefactor -- to restrain Pyongyang.

US National Security Advisor Jake Sullivan said last week that Washington and allies Seoul and Tokyo would "stand up" for stability across the Taiwan Strait, a sensitive waterway separating Taiwan from China.

The three allies also reiterated their commitment to freedom of navigation in the disputed South China Sea.

China has in recent years ramped up military and political pressures on democratic Taiwan, which it claims as its territory.

To reinforce that both the Taiwan Strait and the South China Sea are international waterways, Washington and its Western allies have increased "freedom of navigation" crossings by naval vessels, angering Beijing.

Nuclear-armed North Korea last month put a military spy satellite into orbit, with Seoul saying it did so with Moscow's help, in return for supplying weapons for use in Russia's war with Ukraine.

The satellite launch fractured an inter-Korean military agreement established to de-escalate tensions on the peninsula, with both countries ramping up security along the demilitarized zone separating them.

In June, South Korea deployed fighter jets in response to Chinese and Russian warplanes near its airspace, as the two countries conducted joint air force patrols over the Sea of Japan and the East China Sea.

Military jets from Moscow and Beijing also entered and exited Seoul's KADIZ in November last year, prompting the South to scramble its fighter jets.

Similar to the incident on Thursday, none of them violated South Korea's airspace at that time.