The accident occurred Wednesday in northwestern Russia, Kyodo reported. ITAR-Tass news agency said the accident was probably caused by poor visibility because the sun barely rises this time of year in the area.
The Japanese Foreign Ministry contacted Watanabe's father, Hiroyasu, saying the victim was "almost certainly" Watanabe, 31.
"I do not want to believe it until I see his face," Hiroyasu Watanabe said.
The Japanese Embassy said Watanabe's passport and identification card from Kyushu Sangyo University were found with the body.
Watanabe was 22 when he became the youngest Japanese climber to scale the tallest mountains of all seven continents, Kyodo said.