The South Korean Defence Staff says the incident occurred on Saturday. The plane left the air defence zone shortly after South Korea sent its own fighter jets.
An air defence zone is not the same as a country's national airspace, but an area where the country in question controls and monitors aircraft.
China's Ministry of Defence confirms that the air forces of China and Russia conducted a "strategic air patrol" over the Sea of Japan. The statement makes no mention of South Korea or its air defence zone.
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