It was the first time a Russian military aircraft had violated South Korean airspace, an official at the South Korean Ministry of National Defence said in Seoul.
Two Russian bombers and two Chinese bombers entered the Korea Air Defence Identification Zone (KADIZ) together early on Tuesday, the defence ministry said.
A separate Russian early warning and control aircraft later twice violated South Korean airspace over Dokdo - an island that is occupied by South Korea and also claimed by Japan, which calls it Takeshima - just after 9 a.m. (midnight GMT Monday), according to the South Korean military.
Russia's defence ministry denied that its strategic bombers had violated South Korean airspace, the RIA news agency reported.
South Korean military planes crossed the path of Russian bombers and did not communicate with them, Russia's defence ministry said, according to RIA.
This was not the first time that South Korean pilots had interfered with a Russian military flight over neutral waters, the ministry said.
China's foreign ministry said South Korea's Air Defence Identification Zone was not territorial airspace and all countries enjoyed freedom of movement in it.
South Korea's top security adviser, Chung Eui-yong, lodged a strong objection with Nikolai Patrushev, secretary of the Security Council of Russia, asking the council to assess the incident and take appropriate action, South Korea's presidential office said.
"We take a very grave view of this situation and, if it is repeated, we will take even stronger action," Chung said, according to South Korea's presidential office.
'Tactical action'
South Korea's foreign ministry would summon Russian Deputy Chief of Mission Maxim Volkov and Chinese Ambassador Qiu Guohong on Tuesday to lodge a stern protest and strongly urge them to prevent a recurrence, said ministry spokesman Kim In-chul.
Japan lodged a complaint with both South Korea and Russia over the incident, a government spokesman said.
"We cannot accept these kinds of actions in our territory," the Kyodo news agency cited government officials as saying.
The Russian aircraft was an A-50 airborne early warning and control aircraft, an official at South Korea's Joint Chiefs of Staff (JCS) said, and South Korea scrambled F-15 and F-16 fighter jets in response to the intrusions.
The South Korean jets fired about 360 rounds of ammunition during the incident, the JCS official said.
"The South Korean military took tactical action including dropping flares and firing warning shots," the defence ministry said.
A South Korean defence official told Reuters that the Russian aircraft did not respond in any threatening way.
The Russian aircraft left South Korean airspace but then entered it again about 20 minutes later, prompting the South Koreans to fire more warning shots.
The ministry said South Korean warplanes "conducted a normal response" to the incursion, without giving further details.
