Rural Chinese children are three to six times more likely than city children to die before they turn 5, a study found, highlighting the wide gulf in healthcare for the rich and poor in China.

Pneumonia, birth asphyxia, and premature birth complications are the leading causes of death in children younger than 5, the researchers said in a paper published on Friday in the Lancet.

The team predicted that complications caused by premature birth will soon become the leading cause of childhood death in China as increased access to hospital treatment is cutting the number of deaths from pneumonia.

Led by Igor Rudan at the Croatian Center for Global Health in Split, Croatia, the researchers searched public databases containing information from 1990 to 2008, including 206 long-term studies on the causes of death in children younger than 5.

According to the paper, child mortality rates dived 71 percent from 64.6 to 18.5 per 1,000 live births from 1990 to 2008, but the disparity between child health in the booming cities and the poor countryside remained stark.

"The progress towards reduction of the child mortality rate in China is strongly determined by the degree of socioeconomic development," they wrote.

Chinese government officials say they have been working to address this inequity since 2003, putting into place a modest healthcare insurance system that they hope will help the poorest meet basic medical needs.

In 2009, Beijing launched a new reform, pledging $123 billion over the next three years to provide universal and affordable basic healthcare for its 1.3 billion population.

In an interview with Reuters last week, Chinese Health Minister Chen Zhu said the insurance program has been extended to much of the population.

"Now we are a 1.33 billion population, and 1.23 billion are covered. Some 100 million are not covered, and these are migrants, elderly people and children in cities, and people working for small enterprises," Chen said.

"I hope that next year, a part of these 100 million people who are still uninsured.... will be brought in."

Under China's hukou household registration system, hundreds of millions of migrants from the countryside are unable to obtain residency status that would give them access to healthcare, education or legal protection in the cities where they settle.

China is home to 15 percent of the world's children, but until recently, information on child, infant and newborn mortality was not easily accessible to researchers outside the country and was not included in global estimates of childhood disease.