The South Korean military fired hundreds of warning shots at a Russian aircraft that entered its airspace on Tuesday, defence officials in Seoul said. Two Chinese military aircraft were also said to have violated South Korean airspace.
South Korea's top security adviser lodged a strong objection with Russia after the incident, the presidential office said, and will take stronger action if it happens again.
South Korea's Chung Eui-yong, chief of the National Security Office, sent a strong objection to Nikolai Patrushev, Secretary of the Security Council of Russia, asking them to assess the incident and take appropriate action, the president's office said.
"We take a very grave view of this situation and, if it is repeated, we will take even stronger action," Mr Chung said.
It was the first time a Russian military aircraft had violated South Korean airspace, an official at the Ministry of National Defence in Seoul said.
But Japan, which also scrambled an aircraft to intercept the Russian jet, criticised Seoul for taking action and Moscow for the incursion, saying that the incident took place in its airspace.
"Takeshima is Japan's territory," Foreign Minister Taro Kono told a news conference, referring to an island claimed by both South Korea and Japan. South Korea calls the island Dokdo.
"It is Japan that should take action against the Russian plane that entered its airspace. It is incompatible with Japan's stance that South Korea takes steps on that," Mr Kono said.
The warplane was one of three Russian aircraft to enter the Korea Air Defence Identification Zone (Kadiz) early on Tuesday, one of which entered the zone twice, the defence ministry said.
Two Chinese military aircraft also entered the Kadiz, it said.
Russia’s defense ministry denied its warplanes had entered Korean airspace and instead accused two South Korean F-16 fighter jets of carrying out "unprofessional manoeuvres" by crossing the path of the bombers.
"It was not the first time that South Korean pilots tried unsuccessfully to prevent Russian aircraft from flying over the neutral waters of the Sea of Japan," the ministry said, adding that the Korean jets did not communicate with Russian pilots.
China's foreign ministry responded to accusations it violated Korean airspace by claiming that it was not territorial airspace and all countries have freedom of movement there.
The South Korean government would lodge official complaints about the violations with China and Russia, the defence ministry said.
According to the South Korean military, the single Russian aircraft violated over Dokdo – an island that is occupied by South Korea and also claimed by Japan – just after 9am local time.
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. The South Koreans then scrambled F-15 and F-16 fighter jets in response to the intrusions.
They fired around 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 the Russian aircraft did not respond in any threatening way.
The Russian jet left the South Korean airspace but then entered again about 20 minutes later, prompting more warning shots.
The ministry said South Korean warplanes "conducted a normal response" to the incursion, without giving further details.