China is considering further changes to its family planning laws, Premier Li Keqiang said Sunday, after a relaxation in the "one child policy" failed to see significantly more babies being born.

The ruling Communist Party imposed strict rules in the late 1970s to limit population growth, with most urban couples restricted to a single offspring.

The often brutally enforced policy has been hugely controversial, but officials say it has been a key factor in China's rising prosperity. Now, though, it is leading to demographic problems including a rapidly ageing population and a shrinking labour force.

A relaxation in the regulations in late 2013, allowing couples to have two offspring if at least one parent was an only child, failed to see a marked increase in births.

Li told reporters that Beijing would assess the reform along with "China's economic and social development situation" before any possible change in regulations.

"Both the pros and cons will be weighed," he said, adding that "improvements, adjustments" would only be made in accordance with legal procedures.

Li's comments were measured but were in marked contrast to past official declarations that family planning is a "fundamental national strategy" that cannot be "shaken".

The topic was raised at his once-a-year meeting with journalists at Beijing's Great Hall of the People -- where questions are generally submitted in advance -- by state broadcaster CCTV and seized on by the official news agency Xinhua.

Among its consequences, the one child policy has also created a severe gender imbalance due to a traditional preference for sons.

Nearly 116 boys were born for every 100 girls in China in 2014, while the sex ratio in the total population was 105 men to 100 women.

Senior official Liu Binjie said on Tuesday that China was reviewing the 2013 change after revelations only 470,000 babies were born as a result -- representing one tenth of families newly eligible to have a second child.

Experts have proposed a further loosening of the laws -- and one local family planning official even called last month for a mandatory two-child policy.