Zou Dehuai has collected thousands of photographs related to Japan's invasion during World War II. [Photo provided to China Youth Daily]

A post-90s man has collected thousands of historical photos related to Japan's invasion during World War II, of which at least 2,000 have never been seen before.

Some Japanese soldiers in the photos are said to be from Unit 526 and "chemical troops". The pictures are among hundreds discovered in an old bookstore in Japan by Zou Dehuai's Japanese friend and then posted to China.

Photos collected by Zou show the existence of Unit 526. [Photo provided to China Youth Daily]

Unit 526 was a biological and chemical unit of the Japanese Army. In 1991, two veterans who served in the unit first revealed the hidden truth of its role. Its very existence has been controversial due to a lack of evidence with detailed proof mostly destroyed before Japan's surrender.

According to the photo album, the owner, then 21, joined the Japanese army on March 27, 1944. In the same year he was assigned to Unit 526 in Qiqihar, Northeast China's Heilongjiang province. No signs of experiments involving biological weapons are directly visible in the photos however they prove the existence of Unit 526.

Since the 1950s a large number of chemical weapons have been found in Qiqihar. Four barrels of a toxic agent were discovered on August 4, 2003 which killed or injured 43 people.