Tokyo has lodged a protest with Moscow after a foreign aircraft - believed to be Russian - briefly violated its airspace, prompting Tokyo to scramble four fighter jets.
The foreign ministry made the protest shortly after the plane entered Japanese airspace off the northernmost main island of Hokkaido, near a disputed island chain.
"We made the protest through the Russian embassy in Tokyo," a foreign ministry official said of the incident on Tuesday afternoon.
"The Russian side did not confirm the case, only saying they will check."
The Japanese Air Self-Defense Force scrambled four jets to head off the intruder, which it believed was Russian after analysing its course, a defence ministry official said.
If confirmed, this would be the first time Russian planes have entered Japanese airspace since August 2013, when two Tu-95 Russian strategic bombers were intercepted off the southwestern Okinoshima Island.
Sixteen seconds after entering Japanese airspace, the plane left toward the Kuril island chain, claimed by Tokyo but controlled by Russia, the ministry added.
Soviet troops seized the islands, known as the Northern territory in Japan, just after Japan surrendered in World War II.
The seven-decade-old dispute has hampered trade and prevented Moscow and Tokyo from signing a formal post-war peace treaty.
Foreign Minister Fumio Kishida is considering visiting Moscow next week to discuss a possible trip by Russian President Vladimir Putin to Tokyo later this year, the daily Sankei Shimbun newspaper has reported.
Kishida's planned visit to Moscow was rescheduled in August after Tokyo hit out at Russian Prime Minister Dmitry Medvedev's trip to one of the disputed islands.