Activist Tamaki Matsuoka is challenging Japanese perceptions of the country's war record with a new documentary on the atrocities known as the Rape of Nanking.

Her film, Torn Memories of Nanjing, combines the memories of Japanese war veterans with accounts by Chinese survivors of the massacres of 1937-38, after Japan captured the former capital city of Nanking.

The film was shown at the Hong Kong International Film Festival on Sunday in its first screening outside Japan.

In the documentary, Matsuoka captures former soldiers admitting for the first time to mass rape and to the masscre of unarmed civilians in Nanking, which is now called Nanjing.

This runs counter to the accepted wisdom in her country, where history textbooks gloss over atrocities during Japan's invasion of China, and Second World War veterans are thought to have battled and lost an honourable fight .

"Chinese and Japanese perceptions of this war are totally different," Matsuoka said Tuesday. "That's why this documentary is called Torn Memories of Nanjing. My mission is to help more Japanese people learn the facts," Matsuoka said Tuesday.

Tomokazu Takeda, a young Japanese who helped produce the documentary, said he had no knowledge of the wartime experience of Nanking.