More than nine million children have been "left behind" in China's countryside by parents who have moved to its towns and cities to find work, Beijing said Thursday.

The plight of such children, who are usually looked after by grandparents but sometimes have no guardians at all, is one of the most emotive consequences of China's decades-long economic boom.

Hundreds of millions of migrants have moved from rural areas to take jobs in urban centres, where their children would have limited access to schooling and healthcare under China's household registration system, forcing them to be left with relatives.

They sometimes see their mother and father only once a year.

A government census showed there were a total of 9.02 million "left behind" children in the country, the civil affairs ministry said in a statement on its website.

Nearly 90 percent -- 8.05 million -- lived with their grandparents, 3 percent were cared for by other relatives, and four percent were entirely alone -- almost 400,000 children.

Stories of those unable to fend for themselves periodically rock the nation.

In 2015, four siblings aged between five and 14 who had been left unattended by their parents for months apparently committed suicide by drinking pesticide in the remote southwestern province of Guizhou.

"The flow of migrant workers driven by urbanisation has affected the family unit and many parents lack a real awareness of their responsibilities," Tong Lihua, director of a Beijing-based legal aid organisation for teenagers, told the official Xinhua news agency.

The new 9.02 million official figure was in marked contrast to a previous statistic of 61 million children given by an All-China Women's Federation survey in 2013.

The civil affairs ministry said the decline was largely due to changing definitions, with the women's federation defining "left behind" children as those under 18 with one migrant worker parent, while the new census restricted the category to those under 16 with two migrant parents, or with one migrant parent and the other incapable of guardianship.

The new definition was "more in line with China's national conditions", the ministry said.