An expert is calling for establishing a systemic mechanism to help left-behind girls in China.

"The probability of left-behind girls suffering sexual assault is high. The problem has become one of the main social issues in China," said Lu Shizhen, vice-chairperson of the China Youth and Children Research Association.

In August, a 15-year-old girl and her 13-year-old brother were killed at home in Zhongxin village of Bijie, Guizhou province. Police found that the girl was sexually assaulted before she was brutally murdered. Her parents were migrant workers away from home and the siblings are considered left-behind children.

Last year, 10 villagers were sentenced to jail for repeatedly raping and sexually assaulting a 13-year-old girl in Guangxi province. She is also a left-behind girl.

"Left-behind girls affect their children, China's next generation, which is a serious problem needing more attention," Lu said, adding that the predicaments of left-behind girls affect China's society, children's and women's development, as well as the nation's future.

"The number is striking," Lu said, citing a report in Guangzhou Daily that said 2,500 girls were sexually assaulted in three years, nearly half of them under 14.

"Left-behind girls have a stronger need for family than boys. Psychologically, girls are more sensitive. Family situations such as a mother's absence would be more harmful to girls than boys," said Lu, adding that left-behind girls also shoulder more housework at home.

According to a report by the All-China Women's Federation in 2012, China has about 28 million left-behind girls.

"It is essential to establish a legal policy system for left-behind girls, and also build a mechanism of government-led and social efforts to guarantee enough investment," she said.