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.

The incident, which also involved China and Japan, could complicate relations and raise tension in a region that has for years been overshadowed by hostility between the United States and North Korea.

Two Russian Tu-95 bombers and two Chinese H-6 bombers entered the Korea Air Defence Identification Zone (KADIZ) together early on Tuesday, the South Korean defense ministry said.

A separate Russian A-50 airborne 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 defense ministry said two Tu-95 strategic bombers carried out a planned flight, but denied that they had violated South Korean airspace and said it did not recognize South Korea's KADIZ.

There were no warning shots from the South Korean fighters, the Russian defense ministry said in a statement, which made no mention of any A-50 aircraft.

The Russian ministry said two South Korean F-16 fighter planes carried out "unprofessional maneuvers," crossing the path of Russian bombers and did not communicate with them.

"It was not for 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.

A South Korean defense ministry spokesman did not directly address the Russian accusation of reckless behavior but said that South Korea never said the Tu-95 bombers had violated its airspace.

China's foreign ministry said South Korea's air defense identification zone was not territorial airspace and all countries enjoyed the 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.

REUTERS