SHANGHAI, Apr 24 -- THE only surviving sex slave in Nanjing who came forth to testify about the Japanese invaders' "comfort woman" atrocity was in critical condition after suffering a brain haemorrhage.

Lei Guiying, 79, was forced to be a comfort woman when she was 13 years old during the invasion of China by Japanese troops in the World War II, the Oriental Morning Post reported today. She was the first and only known surviving victim in Nanjing who can confirm the history and fate of the comfort women.

Lei is in the Jiangsu Hospital of Traditional Chinese Medicine where a doctor said she is on the verge of death.

Li Cuilan, Lei's daughter-in-law, said Lei began vomiting from 7pm on Sunday. "She fell unconscious when we sent her to the hospital," Li said.

Su Qinglin, a doctor of the hospital, said the old woman should be observed for 24 hours in the hospital.

A Nanjing residents named Qin Jie, 74, a survivor of the 1937 Nanjing Massacre, donated 2,000 yuan (US$259) for Lei's treatment.

"We will protect these witnesses to protect the history," Qin said.

Lei took a bottle of potassium permanganate when escaping from sexual slavery 64 years ago. The chemical was used by the Japanese to wash and disinfect the women.

The chemical was typically used in sex slave quarters, according to former Japanese soldiers and documents, and can be important evidence to confirm the history of comfort women.

About 200,000 Chinese women were forced into sexual slavery nationwide during the invasion by Japanese troops. Most women have died, and have not come forth for fear of prejudice. Only 84 comfort women have been identified on the Chinese mainland.

Japanese Prime Minister Shinzo Abe has apologized for the wartime sex slavery.

"As Japan's prime minister, I am extremely sorry that they were made to endure such pain. We feel responsible over the situation in which the women had to exist as 'comfort women' and endure such hardship." Abe said.