TOKYO -- Japan said that Russian fighter planes violated its airspace briefly on Thursday, and it scrambled its own military jets in response. The episode caused a flare in tensions in a longstanding territorial dispute that had been relatively quiet in recent months.
Russia denied the accusation, which came on the day each year that is set aside in Japan for rallies supporting the country's claims to an island chain near the northern Japanese island of Hokkaido. The islands have been controlled for decades by the Soviet Union and then Russia.
The Japanese government issued a "severe protest" with the Russian Embassy in Tokyo, and it demanded that Moscow investigate.
The rally and the reported incursion underscore the tangled history of territory in Northeast Asia and the friction it causes. Japan is embroiled in disputes over two separate groups of islands with South Korea and China. Of the three, the conflict over the northern islands has been the least problematic in recent months, while the frequent brinkmanship over the islands claimed by Japan and China has led to fears that cat-and-mouse games among the nations' boats and planes could turn more serious.
Prime Minister Shinzo Abe of Japan was attending one of the yearly rallies on the islands controlled by Russia when he was informed of the claims of the intrusion in Japanese airspace. Just before the news, he said he was "determined to do everything in our power" to resolve the dispute in Japan's favor.
Although Mr. Abe is hawkish and is pushing for more military spending, other prime ministers have also attended rallies over the islands, known as the Northern Territories in Japan and the Southern Kuriles in Russia. Mr. Abe told Russia's president, Vladimir V. Putin, in a phone call in late December that the bond between Japan and Russia was "one of the most promising bilateral relationships" and that his administration would "prioritize strengthening ties" with Russia.
Japan and Russia have argued over the three islands and a group of islets for years, a dispute that has kept the two nations from signing a treaty to end their hostilities in World War II.
Soviet troops seized the islands and islets, which are in rich fishing grounds and are thought to lie near underwater oil and gas reserves, in the final days of the war.
A Japanese Defense Ministry official, Yasunari Oyama, said that four Japanese air force jets scrambled after two Russian jets veered into Japanese airspace for a little more than a minute.
Mr. Oyama said it was not immediately clear whether the episode was intentional or accidental. The planes were flying off the northwest tip of Hokkaido, while the disputed islands are off the northeast tip, and Russian planes have routinely skimmed along Japanese airspace off the west coast since the cold war.
Still, Mr. Oyama said that Tokyo considered Russia's actions "unacceptable." It was the first intrusion by Russian jets into Japanese airspace in five years, he said.
As president in 2010, Dmitri A. Medvedev became the first Russian or Soviet leader to visit the disputed islands. He made it clear that Russia had no plans to give up the territory.
The sparring on Thursday came just days after Japan said a Chinese frigate had aimed a weapon-targeting radar on a Japanese warship in the East China Sea, where the two nations are embroiled in their separate islands dispute. Mr. Abe has called China's actions -- sending patrol boats or planes to the area almost daily -- "dangerous" and "provocative."
Capt. Roman Martov, a spokesman for Russia's eastern military district, said military planes flew a planned exercise in the area of the disputed islands but did not enter Japanese airspace, according to RIA Novosti, the state-owned Russian news agency.
"The Pacific Fleet's naval aviation flies in this region regularly, and in strict compliance with international law, using airspace without violating the borders of other states," the news agency quoted Captain Martov as saying. He said airplanes practiced antiship maneuvers in the Sea of Okhotsk on Thursday, as well as flying in difficult weather.
Andrew Kramer contributed reporting from Moscow.
This article originally appeared in The New York Times.