It seems to have started with Putin's offer to sign the peace treaty to end the formal WWII hostilities - by the end of the year. The Russian president said this during an economic forum session in Vladivostok, Russia's Far East.
'Both Abe and Putin say they want a solution to the dispute. The Japanese leader has said a deal would unlock trade and investment from Japanese companies for Russia, which is isolated from Western investment because of sanctions. But for Putin, who casts himself publicly as a fierce champion of Russia’s national interests, giving up the islands would risk a political backlash at home', Reuters wrote.
The news agency added that 'after Putin had made the offer, Japan’s Chief Cabinet Secretary Yoshihide Suga said: 'I don’t want to comment on what President Putin said...However, our position that the Northern Territories issue is resolved before any peace treaty remains unchanged'.'
Russia and Japan have been involved in the territorial dispute over Kurile islands since the end of the Second World War.