Two Russian fighters briefly intruded into Japanese airspace off the northern tip of the island of Hokkaido, defence ministry says.
The Japanese air force was scrambled during the incident, which lasted less than a minute, an official said.
He said it was not immediately known whether the airspace violation was intentional or accidental, but that it was "extremely problematic."
He says the last intrusion by Russian jets in Japanese airspace was in February 2008.
The two countries are locked in a long-running dispute over a chain of islands in the Pacific.