With the world's biggest population straining scarce land, water and energy resources, China has enforced rules to restrict family size since the 1970s. Regulations vary but usually limit families to one child, or two in the countryside.

"We want incrementally to have this change," Vice Minister of the National Population and Family Planning Commission Zhao Baige told reporters in Beijing.

"I cannot answer at what time or how, but this has become a big issue among decision makers," Zhao said. She added that the current plan was to study the issue seriously and responsibly, but avoid sudden changes that might cause a spike in births.

Teams studying the issue would have to consider the strain of China's huge population on its scarce resources, popular attitudes, and how much of a social net China can afford to provide without the traditional reliance on large families to care for the aged, she said.

Surveys show that 60 percent of Chinese younger than 30 want two children, and only a "very small" number want more than three, Zhao said.

The average number of children that would be born to a woman over her lifetime has decreased to 1.8 in China today, from 5.8 in the 1970s, and below the replacement rate of 2.1.

China says its policies have prevented several hundred million births and boosted prosperity, but experts have warned of a looming social time-bomb from an ageing population and widening gender disparity stemming from a traditional preference for boys.

Still, the government has previously expressed concern too many people are flouting the rules.

Normally, between 103 and 107 boys are born for every 100 girl infants, but in China, 118 boys are born for every 100 girls, Zhao said. Experimental policies include trying to improve women's welfare and girls' access to schooling.

State media said in December that China's population would grow to 1.5 billion people by 2033, with birth rates set to soar over the next five years.

Officials have also cautioned that population controls are being unravelled by the increased mobility of China's 150 million-odd migrant workers, who travel from poor rural areas to work in more affluent eastern cities.

China has vowed to slap heavier fines on wealthy citizens who flout family planning laws, in response to the emergence of an upper class willing to pay standard fines to have more children.

Reuters