It could take up to 15 years for China's gender imbalance to sort itself out, the country's top family planner said on Tuesday, admitting that three decades of strict population policies had contributed to the problem.

In 2005, there were 118 boys to every 100 girls born in China as wider use of ultrasound and easy availability of abortions compounded a traditional preference for boys. In some parts of the country, the imbalance is as high as 130 to 100.

"There are many reasons for the gender imbalance, and the first is the existence for thousands of years of a deep-rooted traditional view that men are worth more than women," said Zhang Weiqing, head of the National Population and Family Planning Commission.

"Of course, there is a certain relationship between the imbalance and China's strict family planning policy," he told a news conference.

"It has only exacerbated the problem, but that is not to say that having this policy has necessarily caused the large imbalance," Zhang added.

He said South Korea, Taiwan and Pakistan had similar problems even though they lacked China's severe controls.

Zhang said the government would make more effort to raise women's place in society and protect baby girls, as well as to crack down on illegal gender selection tests and sex-selective abortions.

"Solving this issue is rather difficult, and we may have to wait 10 to 15 years for the proportion to balance out."

China is the world's most populous nation with about 1.3 billion citizens at the latest estimate.

Last year it scrapped plans to make sex-selective abortion - which is already banned - a crime. Experts have said such a step would more effectively deter parents from aborting baby girls.

Zhang did not elaborate on what specific legal methods the government would use to deal with the problem.

But he defended the national population plan - which he said had prevented 400 million births in the past 30 years - and said it was wrong to think of it as a "one-child" policy.

The reality these days was much more complex, Zhang said, with different provinces making their own adjustments to the rules as the situation demanded.

Only just over one third of the population was strictly limited to just one child, but in 19 provinces rural couples were allowed to have another baby if their first was a girl, the minister said.

And in five provinces, including the southern island of Hainan and Yunnan in the south-west, all rural parents were allowed to have two children.

"People may ask, why is the policy so complex? It is decided by China's national situation. There are different policies which must stem from China's very unbalanced economic and social development," Zhang said.