South Korea urged Japan Thursday to reflect on its World War II biological and chemical warfare unit responsible for some of its most notorious war crimes, after a militaristic photo of Prime Minister Shinzo Abe evoked the "painful memories of Unit 731." 

Unit 731 was the Japanese Imperial Army's covert biological and chemical warfare research and development unit, carrying out a special project that used prisoners of war for experiments.

Historians say the unit, based in Harbin, held prisoners from China, Korea and the Soviet Union.

Abe held a militaristic photo opportunity on Sunday with the photo showing a smiling Abe while sitting in the pilot's seat of a Japanese Air Self-Defense Force training jet emblazoned with the number 731. Although it remains unclear whether there was any intent behind the use of a jet with the "731" marking, the photo sparked outrage from South Korean media.

When asked about the Abe's photo in question, Seoul's foreign ministry spokesman Cho Tai-young replied that South Korea expects "the Japanese side to reflect on painful memories of Unit 731 and its historical background."

"Unit 731 is one of the atrocities committed by the Japanese Imperial Army," Cho said, adding that the unit caused "enormous pain and damage to relevant people in neighboring countries."

Japan has not officially acknowledged the atrocities of Unit 731. (Yonhap News)