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.
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