Two Tu-95 bombers entered Japanese airspace near Minamidaito Island in Okinawa Prefecture on Thursday morning for about three minutes, and one later re-entered the airspace for about two minutes near Hachijo Island in the Izu island chain, the ministry said. It added no dangerous maneuvers were observed.

The planes were later seen flying southward from the Sea of Japan through the Tsushima Strait between Japan and South Korea in the morning.

A "stern protest" against Russia was lodged over the incident, the Japanese Foreign Ministry later said. Russia insisted that the planes did not violate Japan's airspace but flew over international waters during a 14-hour flight.

"Two strategic bombers Tupolev-95MS of Russia's Aerospace Force flew a routine mission in the airspace over the international waters of the Sea of Japan and the East China and South China seas as well as the western part of the Pacific Ocean," the Russian Defense Ministry said in a statement on Friday. "The flight was carried out in strict compliance with the international rules of using international airspace, without any violations of the borders of other states."

Japan did not report the number of fighter planes it used to divert the Russian planes.

A report by Japan's Defense Ministry said its fighter jets had scrambled nearly 1,000 times against foreign aircraft approaching Japanese airspace in the year ending on March 31. Over 340 incidents involved Russian aircraft and 638 involved Chinese planes, it said.