However, "there was no invasion of airspace", the South Korean Joint Chiefs of Staff said in a statement.

"Our military received a reply from the Chinese side that it was... normal training through the South Korea-China direct communication network."

The JCS said it had evaluated the situation to be a Russia-China joint exercise, and that the aircraft flew inside the zone for around 10 minutes.

South Korea dispatched F-15 and F-16 fighters along with an aerial refuelling tanker as a "normal tactical measure" as the Russian and Chinese planes approached the zone, the JCS added.

The incursion happened northeast of Dokdo -- islets that are also claimed by Japan, which calls them Takeshima.

Air defence identification zones are outside a country or territory's airspace, in which foreign aircraft are monitored for security reasons.

However, they are self-declared and technically international airspace.

In 2019, South Korean fighter jets fired more than 400 warning shots and Japan complained to Moscow after a Russian military plane violated airspace near the disputed islets.

Russia said at the time that it did not recognise such zones.