Japan and South Korea have scrambled jets in response to a joint Russia-China air patrol.
The Russian defence ministry says four bombers, supported by fighter jets, patrolled a pre-planned route over the Sea of Japan and the East China Sea.
South Korea says its jets fired flares and machine-gun warning shots when Russian planes intruded.
Japan has protested both to Russia and South Korea over the incident.
The alleged incursion happened over the disputed Dokdo/Takeshima islands, which are occupied by South Korea but also claimed by Japan.
South Korea says Russian and Chinese planes entered the Korea Air Defence Identification Zone (KADIZ) on Tuesday morning – and that a separate A-50 Russian warplane twice violated its airspace near the islands.
Russia denies the accusation.
Russian and Chinese bombers and reconnaissance planes have occasionally entered the zone in recent years, but this is the first incident of its kind between Russia and South Korea.