China's vast number of rural migrant workers have become the backbone for rapid urban development, but many of their children are left at home in poor rural areas without parental care and protection.

"Local police have been told to be on high alert for crimes against 'left-behind' children. Some of the children have been abducted, robbed or forced to commit crimes," said Wu Dongli, director of MPS's security management bureau.

The ministry will strengthen rural patrols to prevent or reduce crimes against these children, Wu said.

"No matter how insignificant a case may be, police are required to promptly open a file and conduct an investigation," Wu said.

Statistics show the number of "left-behind" rural children have reached 20 million in China and the number is still on the rise as their parents continue to flood into cities.