Four Russian and four Chinese military aircraft entered the zone off South Korea's east and south coasts around lunchtime Tuesday, Seoul's Joint Chiefs of Staff (JCS) said in a statement.

An air defence identification zone is an area wider 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.

The South Korean military "identified the Chinese and Russian jets before their entry into the air identification zone", the JCS said.

"We deployed air force fighters to conduct tactical steps in preparation in case of an emergency," it added.

The eight foreign jets did not violate Seoul's airspace, the JCS said.

The incident comes after the defence ministers of South Korea, Japan and the United States on Saturday agreed to set up real-time data sharing on North Korean missile launches by the end of the year.

South Korea has supported Western sanctions on Russia over the war in Ukraine and has sent humanitarian aid to Kyiv. But it is bound by its own regulations prohibiting the supply of lethal aid to countries in conflict.