There is only one way 89-year-old Zhang Xiantu can cope with the horrors that haunt her from the time she spent in sexual slavery during the Japanese occupation of China.

“When I start to remember,” says Zhang. “I force myself to forget.”

Zhang is among the subjects of "Twenty Two", a documentary by Chinese filmmaker Guo Ke that sets out to tell the story of what were the last 22 surviving “comfort women” living in China. Three have died since filming finished in mid-2014.

An estimated 200,000 women were forced into sexual slavery by the Japanese military from 1931 to 1945.

"Twenty Two" made its world premiere at the Busan International Film Festival, where it is in competition for the event’s main Wide Angle documentary competition, in a week where tensions between Japan and China have once again flared over the issue of “comfort women.”

Japanese newspapers have reported authorities complained to UNESCO following China’s application that the “comfort women” story be entered into the organisation’s Memories of the World register, which was set up in 1992 to preserve “documentary heritage.”

A panel will announce their decision on what is included in the register on Friday.

'Lost to history'

Last month China staged massive commemorations nation-wide to mark the 70th anniversary of the end of its conflict with Japan and has long fought for what is sees as a “sincere apology” to victim nations.

“I did not want these women lost to history,” Guo said on the sidelines of the festival in Busan.

“No one can fully understand what they must have gone through but I wanted to put their stories down before they are lost to history and I wanted to world to see that these women are heroes.”

With the help of the Shanghai-based Research Centre for Chinese Comfort Women, Guo took two years to track down the survivors and film them in their rural Chinese villages.

Some were willing to talk about the past and some refused, while age and infirmity rendered some unable to respond at all.

Among their number are those kidnapped from other territories occupied by Japan at the time, including the Korean peninsula.

“These women were often brought to China from other parts of Asia and then when the war ended, they just didn't want to go home to face their families again after what they had lived through,” says Guo.

We see the remains of the cells and caves in which the women were held, some of which are in walking distance from the homes in which they still live.

There are those, such as 90-year-old Lin Ailan who lives in a tiny village in China’s southern Hainan province, and was captured when fighting against the Japanese forced into sexual slavery.

She recalls how the Japanese suggested she marry one of them and she thought “if I did I could cut his throat.”

Guo sets his cameras up to follow the woman as they get on with everyday life, and often simply focuses on their faces as they sit in silent contemplation.

The director says at such times he wants his audience to think about what these women must still be carrying around inside.

“Some of them talk but there is always a time when the memories become too much,” he says.

Searching for peace

Guo’s first turned to the topic with the short film "Thirty Two" in 2013, after hearing of the plight of Wei Shaolan, who was enslaved at 24 and escaped captivity while pregnant and went on to raise a son fathered by a Japanese soldier.

“These women could be our grandmothers – yours and mine. I think it is wrong for history to forget them,” he says.

In "Twenty Two", Guo also interviews some of the people who have over the years taken on the women’s cause, from a Korean photographer, to a Japanese student nurse, to a farmer-turned lawyer who has fought on their behalf for some sort of financial compensation.

That fight, Guo says, is now over as the women have collectively decided it is no longer worth the effort.

“They want now to be left with their dignity as they tried to find peace,” he says.

Censorship issues mean it will not get clearance to screen in China but the director hopes to show the film internationally.

“It is not a topic that China likes to discuss but I would like for the rest of the world to give these women honour and respect,” he says.

The winner of BIFF’s documentary section will be announced on Saturday.