MOSCOW/SEOUL (Reuters) - Russia carried out what it said was its first long-range joint air patrol in the Asia-Pacific region with China on Tuesday, a mission that triggered hundreds of warning shots, according to South Korean officials, and a strong protest from Japan.

The flight by two Russian Tu-95 strategic bombers and two Chinese H-6 bombers, backed up by a Russian A-50 early warning plane and its Chinese counterpart, a KJ-2000, marks a notable ramping-up of military cooperation between Beijing and Moscow.

That is something likely to worry politicians from Washington to Tokyo and could complicate relations and raise tension in a region that has for years been overshadowed by hostility between the United States and North Korea.

While troops and naval ships from Russia and China have taken part in joint war games before, they have not, according to Russia’s Ministry of Defence, conducted such air patrols in the Asia-Pacific region together until Tuesday.

“The joint patrol was carried out with the aim of deepening Russian-Chinese relations within our all-encompassing partnership, of further increasing cooperation between our armed forces, and of perfecting their capabilities to carry out joint actions, and of strengthening global strategic security,” the ministry said in a statement.

Seoul and Tokyo, who both scrambled jets to intercept the Russo-Chinese mission, accused Russia and China of violating their airspaces, an allegation Moscow and Beijing denied.

South Korean warplanes fired hundreds of warning shots toward the Russian A-50 military aircraft, defense officials in Seoul said, saying it was the first time a Russian military aircraft had violated South Korean airspace.

Moscow denied all of those assertions.

In Washington, the Pentagon said it supported South Korea and Japan’s responses.

“The United States strongly supports our ROK and Japanese allies and their responses to air space incursions by Chinese and Russian aircraft,” Pentagon spokesman Lieutenant Colonel Dave Eastburn said.

“The (Defense Department) is in close coordination with our ROK and Japanese allies about these events, and will continue to monitor activities as they follow up with their Russian and Chinese counterparts in diplomatic channels,” Eastburn said.

Dmitri Trenin, director of the Carnegie Moscow Center and a former colonel in the Russian army, predicted Russian-Chinese joint air patrols would soon be common in the region.

“Such patrols will become a regular feature under a new agreement soon to be signed between Moscow and Beijing,” Trenin said on Twitter. “Russo-Chinese entente grows thicker.”

Lieutenant-General Sergei Kobylash, commander of Russia’s Long-Range Aviation, said in televised comments that the Russian planes had been airborne for 11 hours and covered around 9,000 km (5592 miles).

Foreign fighter jets had escorted them on 11 separate occasions, he added.

The South Korean defense ministry said the Russian and Chinese bombers had entered the Korea Air Defence Identification Zone (KADIZ) together early on Tuesday.

The separate Russian A-50 airborne early warning and control aircraft later twice violated South Korean airspace over Dokdo - an island that is controlled by Seoul and claimed by both South Korea and Japan, which calls it Takeshima - just after 9 a.m. (midnight GMT Monday), according to the South Korean military.

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Russia’s Defence Ministry said it did not recognize South Korea’s KADIZ, while the Chinese Foreign Ministry said the area was not territorial airspace and that all countries enjoyed freedom of movement in it.

South Korean fighters did not fire any warning shots toward Russia’s two bombers, the Russian defense ministry said in a statement, which made no mention of any A-50 aircraft.

It accused the two South Korean F-16 fighter planes of carrying out “unprofessional maneuvers,” of crossing the path of the Russian bombers, and of not communicating with them.

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If the Russian pilots had felt any threat to their safety, their response would have been swift, it added.

Russia has handed South Korea’s military attache in Moscow a note protesting what it called “the illegal and dangerous actions” of Seoul’s pilots, Kobylash said.

A South Korean defense ministry spokesman did not directly address the Russian accusation of reckless behavior.

South Korea’s top security adviser, Chung Eui-yong, lodged a strong objection with Nikolai Patrushev, secretary of Russia’s Security Council, 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.

South Korea’s Foreign Ministry summoned Russian Deputy Chief of Mission Maxim Volkov and Chinese Ambassador Qiu Guohong to lodge a stern protest and strongly urge them to prevent a recurrence, said ministry spokesman Kim In-chul.

Separately, Japan, which said it had also scrambled fighter aircraft to intercept the Russian and Chinese planes, lodged a complaint with both South Korea and Russia over the incident, Chief Cabinet Secretary Yoshihide Suga said.

Tokyo criticized South Korea for taking action against a Russian plane over what Japan says is its airspace.

“In light of Japan’s stance regarding sovereignty over Takeshima, the fact that the South Korean military aircraft carried out warning shots is totally unacceptable and extremely regrettable,” Suga told reporters.

The South Korean jets loosed about 360 rounds of ammunition during the incident, an official at South Korea’s Joint Chiefs of Staff (JCS) said.

“The South Korean military took tactical action including dropping flares and firing warning shots,” the South Korean Defence Ministry said.

A South Korean defense official told Reuters a Russian A50 plane left South Korean airspace but then entered it again about 20 minutes later, prompting the South Koreans to fire more warning shots.