Keiichi Kurogi is one of dozens of men in Japan who offered to join an "international legion" to fight Russian invaders after Ukraine's President Volodymyr Zelenskyy called for volunteers.

Kurogi, a 39-year-old office worker who lives in southwestern Japan, said he rang the Ukrainian Embassy on Monday after seeing its plea for volunteers on Twitter.

"When I saw images of elderly men and women in Ukraine holding guns and going to the front, I felt I should go in their place," he said.