A South Korean F-15K was one of the jets that were scrambled (Picture: US Department of Defense).
South Korea says it fired more than 350 rounds of warning shots at a Russian military aircraft after it entered its airspace twice on Tuesday.
Three Russian military planes initially entered South Korea's air defence identification zone off its east coast before one intruded in South Korean airspace, the country's defence ministry said.
South Korean F-15 and F-16 fighter jets then scrambled to the area and fired 10 flares and 80 rounds from machine guns as warning shots, a ministry official said.
The Russian A-50 aircraft left the area but returned three minutes later and again violated airspace.
Officials said the South Korean fighter jets replied by firing a further 10 flares and 280 rounds as warning shots.
In total, 360 rounds of ammunition were fired.
The airspace the Russian warplane violated was above a group of South Korean-held islands that have been at the centre of territorial disputes between South Korea and Japan.
The three Russian planes were accompanied by two Chinese bombers.
South Korean officials said it was not immediately known whether the nations had done so deliberately.
South Korea's presidential national security adviser, Chung Eui-yong, was quoted by the presidential office as saying the incident was being taken "very seriously".
Mr Chung also warned South Korea will take "much stronger" measures if a similar incident happens again.
The incident was the first time a foreign military plane had violated South Korean airspace since the end of the Korean War in 1953, according to an official.
In 1983, a Soviet air force fighter jet fired an air-to-air missile at a South Korean passenger plane that strayed into Soviet territory, killing all 269 people on board.
Relations between the two nations have since gradually improved.
Chinese planes have occasionally entered South Korea's air defence identification zone in recent years.
South Korea has summoned China's ambassador and defence attache.