South Korea says it fired warning shots after Russian military planes entered its airspace.
Fighter jets were also scrambled in response.
According to the Defence Ministry in Seoul, it's the first time an aircraft from Moscow has violated the country's territorial sky.
It is planning to summon officials from the Russian Embassy to register formal protests.

China-North Korea bridge opens 12 years after completion
DR Congo Ebola death toll passes 4,400 in record-breaking outbreak
Saudi-led coalition intercepts drone over Eastern Region
Ukraine accuses SpaceX of crippling drone missions as ties with US worsen
Kenya reports second Ebola case, family member of first patient
