"We confirmed that Russian military bombers and fighter jets flew over the high seas of the Sea of Okhotsk and the Sea of Japan yesterday, and we scrambled Air Self-Defense Force fighter jets" in response, Yoshimasa Hayashi told reporters.

"It is difficult to say clearly what the purpose of the flight was... but the Russian military has been active on an ongoing basis in areas surrounding Japan," he said.

Tokyo has raised the issue with Moscow in the past through diplomatic routes, including regarding a Russian fighter jet's intrusion into territorial airspace in September, which Russia denied according to media reports.

"We will continue to monitor (the situation) closely and do our best to take measures in patrolling and responding to airspace incursions," Hayashi said.

Russia's defense ministry reportedly announced that its long-range strategic bombers flew over international waters in the Sea of Japan and the Sea of Okhotsk.

The Russian ministry released a video on Thursday of two Tupolev-95 aircraft conducting what it called a regular flight, Japan's public broadcaster NHK said.

The ministry said the bombers were escorted by other aircraft including fighter jets, according to NHK.

The Russian ministry reportedly noted that pilots of long-range bombers regularly fly over international waters in oceans and seas -- such as the Arctic, North Atlantic and Pacific oceans -- and that all flights are carried out in compliance with international law.