China, the world's most populous country with more than 1.3 billion people, will maintain its strict "one-child" policy, state media said Sunday, despite calls for the rules to be relaxed.

The population control measure has prevented almost half a billion births since it was introduced in 1979, Chinese experts claim.

But it has turned into a demographic time bomb as the population ages, storing up huge economic and social problems for the country as well as fostering a gender imbalance.

"Over-population remains one of the major challenges to social and economic development," Li Bin, director of the State Population and Family Planning Commission told Xinhua, adding China's population will hit 1.45 billion in 2020.

"Maintaining and improving the existing family planning policy and keeping a low reproduction rate, along with addressing the issues of gender imbalance and an ageing population, will be the major tasks in the future," he said.

His remarks came as the world's population is expected to hit seven billion on Monday, according to the United Nations.

Critics blame the "one-child" policy for creating gender imbalances -- sex-specific abortions are common and female infanticide and the abandoning of baby girls have also been reported.

The policy also puts huge pressure on only children to support their parents and two sets of grandparents.

Guangdong in southern China asked Beijing in July for permission to partly relax the policy and allow couples where just one parent is an only child to have a second baby.

But Zhang Feng, director of the province's population and family planning commission, said there would be "no major adjustments to the family planning policy within five years."

Li of the family planning commission defended the policy, saying China's population would have hit 1.7 billion without the requirement.