A retired senior diplomat has urged Prime Minister Shinzo Abe's government to accelerate negotiations on a territorial row with Russia, saying the "window of opportunity" to resolve the long-standing issue is only likely to remain open until "the end of this year."

Kazuhiko Togo, who used to be in charge of talks over four Russian-held islands off Hokkaido, said Wednesday at a press event that it is "nearly criminal" that Japan has not taken any action since last March, when then-Russian Prime Minister Vladimir Putin expressed his readiness to finally settle the dispute.

"It is Japan which is asking very seriously to change the status quo," Togo, a former director general of the Foreign Ministry's European Affairs Bureau, said in Tokyo.