South Korea has lodged a complaint with the Chinese and Russian defence attaches based in the country, a day after their warplanes entered its air defence zone, BBC reported.

Seoul said it sent up fighter jets to "take tactical measures in preparation for any emergencies" after seven Russian and two Chinese military aircraft "briefly entered" the zone on Tuesday, but noted they "did not violate" South Korea's airspace.

Some countries delineate air identification defence zones, wherein they require foreign planes to identify themselves. These are not part of sovereign airspaces under international law, according to BBC.