Japan's defence minister said Sunday there have been an "abnormal" number of flights by Russian military aircraft close to Japanese islands in recent days.

The country's air defence force scrambled fighter jets for seven days in a row through Saturday after spotting Russian military planes flying along the Japanese archipelago, according to the defence ministry.

On Friday six Russian TU-95 bombers were seen flying two by two, with one pair moving around the Okinawan islands and then going north along Japan's Pacific coast.

The two other pairs flew over the Sea of Japan (East Sea). None of the flights intruded into Japanese airspace.

"They are continuing flights, which we deem as abnormal and were unseen even in the Cold War era," Defence Minister Itsunori Onodera told reporters after attending a ceremony in Okinawa to inaugurate an airborne early warning squadron.

The minister, according to footage on public broadcaster NHK, added that Russia's intentions were unclear and Japanese defence officials based in Russia have been trying to check them.

"We are closely monitoring the situation," Onodera said.