Japan's foreign ministry says that two Russian fighter jets have violated Japanese airspace near the northern island of Hokkaido, a charge that Russia has denied.
Tokyo said that it scrambled combat fighters and lodged a protest in response to Thursday's alleged intrusion off Hokkaido. The planes were detected within Japanese airspace for just over a minute, initial reports said.
The alleged intrusion on Thursday came on Japan's "Northern Territories Day", when rallies are traditionally held calling for the return of disputed islands off Hokkaido.
It was the first such intrusion in five years, a ministry official said.
Russia has issued a denial, saying that while flights near the border were common, the international lines were strictly respected.
"Flights by the air force of the Pacific Fleet take place regularly in this region, in strict adherence to the international rules, without violation of state borders," Roman Martov, the spokesperson of the military command's eastern district, said in a statement to Russian news agencies on Thursday.
Former Japanese Prime Minister Yoshiro Mori is expected to visit Moscow later this month to discuss territorial matters.
Russian Prime Minister Dmitry Medvedev landed on the remote island chain in July, prompting protests from Tokyo.
Medvedev's trip to the disputed islands in Russia's Far East - known as the Southern Kuriles in Russia, but as the Northern Territories in Japan, prompted Japan's ministry of foreign affairs to complain.
Unlike Japan's dispute with China over islands in the East China Sea, which are near potentially vast maritime oil and gas reserves, the feud with Russia has more to do with the legacy of World War II.
Soviet soldiers seized the islands at the end of the war and the territorial row has weighed on diplomatic relations ever since, precluding a formal peace treaty.