Deep in the Chinese countryside, the lives of Liang Yuxiu and her brother Zhaolu epitomise the cost of decades of breakneck economic growth.

Aged 10 and 12, they live with their grandparents, helping tend their rice crop at weekends and boarding at school during the week.

Their father is dead and two years ago their mother left Xianghe -- one of the poorest areas of Guangxi, among China's most deprived regions -- in search of work.

"At home, I like watching war dramas on grandma and grandpa's TV, but only after I've finished the day's work in the rice field," says Zhaolu.

Zhaolu and Yuxiu are among the estimated 61 million "left behind" children -- about the population of Italy -- in China's countryside whose parents have moved to the cities to find jobs, or died.

The labour of hundreds of millions of migrant workers has helped achieve China's transformation from an overwhelmingly agrarian society under orthodox Communism to the world's second-largest economy.

But China's "hukou" system of residency permits denies the children of those who move equal access to education and healthcare, and they pay a lonely price.

Most are raised by their grandparents or other family members, and state media report that more than three percent are simply left on their own, citing statistics from the All-China Women's Federation.

Last month four siblings aged five to 13 whose parents had both left home died after drinking pesticide in what state media described as a suicide pact.

"Thanks for your kindness, but it is time for us to go," read a note found in their house in Guizhou province, according to the official Xinhua news agency.

The deaths sparked widespread public sympathy and prompted Premier Li Keqiang to call for "an end to such tragedies".

- 'Three is enough' -

Every Monday, Yuxiu and her brother negotiate narrow, muddy paths for 30 minutes to a road to catch a bus for their hour-long ride to school.

The run-down building, in a village surrounded by green hills and karst rock cliffs, has rusty metal gates and many of its 400 pupils are "left behind".

Li Dandan lost both her parents in a car accident in December, and has since lived with her only remaining grandmother, who relies on a monthly pension of just 300 yuan ($48).

Her short hair tucked under a pink headband, Dandan, 11, walks for more than a hour over hard mountain roads to school to save the four yuan bus fare.

"Dandan is extremely thoughtful," said her uncle. "Her grandmother gives her three yuan a week, but when she decides to give five, Dandan refuses stubbornly and says: 'No grandma, three is really enough'."

More than 82 million people in China were still living on less than about $1 a day at the end of 2013, a senior official said last year.

- 'I often feel guilty' -

Most of the rural schools that cater to "left behind" children are poorly equipped, said Wei Jixue, a teacher in Chongshan village, also in Guangxi, southern China.

"The biggest problem for the children is drinking water," he said. "We drink rainwater for lack of a better option, but this becomes difficult during the dry season," he added, pointing to pupils lining up at a tank.

The 70 students at Wei's school pack into a single classroom, all grades mixed together, and the courtyard is surrounded by a wall of concrete blocks.

On the building's fa�ade hangs a banner in red Chinese characters: "Love your country, love your work, work hard and innovate!"

But "left behind" children seem more preoccupied with helping their families than by schoolwork or games.

Liang Yongyao's mother died three years ago and his father has suffered a stroke, leaving him with difficulty walking and sometimes lacking the strength to work in the fields in Xianghe.

"It's exhausting to plant maize... but my father only allows me to do a little part of the work," said the 12-year-old. "He always works more than I do.

"I often feel guilty. I watch him work himself to death, and I don't know what to say to him. Sometimes I feel like a burden."