South Korea said its fighter jets fired hundreds of warning shots at a Russian military aircraft that twice entered its airspace over a group of small islands off its east coast Tuesday. 

Seoul’s Defense Ministry said its jets scrambled after a Russian A-50 early warning and control aircraft violated its airspace over the islands, which are controlled by South Korea but claimed by Japan. Tokyo said it also scrambled fighters in response to flights by Russian and Chinese aircraft.

The South Korean fighters fired 80 warning shots the first time and a further 280 shots when the aircraft returned a few minutes later, according to the Defense Ministry. Seoul said it was the first time a Russian military plane had violated its airspace, and experts said the incident complicated simmering regional tensions.

The waters of East Asia are a patchwork of competing sovereignty claims. Japan and South Korea often scramble fighters when Chinese military planes enter what they consider to be their airspace, but experts said it was very unusual for warning shots to be fired. It was also strange that the Russian plane allegedly returned after the first shots were fired, they said.

Earlier, two Russian bombers and two Chinese bombers entered South Korea’s self-declared Air Defense Identification Zone, Seoul said. 

Then the Russian A-50 Airborne Warning and Control System (AWACS) plane flew close to the islands known in South Korea as Dokdo and in Japan as Takeshima, first entering airspace over the islands at 9:09 a.m. local time and then again at 9:33 a.m., staying for a total of seven minutes.

South Korea summoned the Russian and Chinese ambassadors to lodge formal protests Tuesday, while Seoul’s national security adviser, Chung Eui-yong, also delivered a warning to his Russian counterpart, Nikolai Patrushev, regarding the intrusion. 

“We view this incident very seriously and will take a stronger measure if such an act gets repeated,” Chung was quoted as saying by presidential spokeswoman Ko Min-jung.

Russia’s Defense Ministry denied that its bombers had entered South Korean airspace or been shot at, but its statement made no mention of the A-50 aircraft referred to by Seoul. 

Moscow said its bombers were flying above neutral international waters and remained more than 25 kilometers (15 miles) away from the disputed islands. It added that it does not recognize South Korea’s Air Defense Identification Zone.

Russia also said that two South Korean F-16s approached the Russian bombers and “conducted unprofessional maneuvers, crossing the course of the Russian aircraft and jeopardizing their safety,” failing to establish communications with the crews of the Tu-5MS bombers, and firing flares, according to state news agency Tass.

Michael Bosack, a special adviser at the Yokosuka Council on Asia-Pacific Studies in Japan, said that could have been human error by the Russian pilots — or a deliberate attempt to “challenge South Korean airspace claims and to test the boundaries of the response.”

Communications problems between the two sets of pilots could have contributed to the rapid escalation of the incident, but it was also possible that Russian pilots had understood but ignored South Korean warnings — or that the South Korean pilots had jumped the gun. 

“If either side were to put out audio or visual records of the incident, we would have a clear understanding of the conditions that led to the escalation,” Bosack said. “Diplomatically, the important thing will be to see the responses from all interested parties.”

Jeffrey Hornung, a political scientist at the Rand Corp., said the incident clearly looked intentional and planned by Moscow and Beijing, and was a headache for South Korean President Moon Jae-in.

“It is beyond imagination to believe that it is merely coincidence for both China and Russia to do this on the same day,” he said. “At a time that the Moon administration has been pursuing a policy of engagement to denuclearize nondemocratic North Korea while enabling relations with democratic Japan to nose-dive to historic depths, I fear its undemocratic neighbors are taking advantage of Seoul’s already burdened foreign policy agenda to sow further problems for Seoul,” he said. 

South Korea and Japan, both U.S. allies, have become embroiled in a dispute stemming from historical grievances that is spilling over into economic retaliation.

The Japanese government also lodged a protest against both South Korea and Russia, Japan’s Kyodo News reported, citing government sources.

Japan said it “cannot accept these kinds of actions in our territory,” Kyodo reported.

It issued a map of the flight paths of the Russian and Chinese bombers, which it said had not entered Japanese airspace, and of the Russian A-50 AWACS plane, whose path twice crossed the outer edges of the airspace above the disputed islands as it completed a U-turn, turning from the south to fly back north.

Min Joo Kim in Seoul, Anton Troianovski in Moscow and Akiko Kashiwagi in Tokyo contributed to this report.