BEIJING - Transferring blood overseas for fetal sex determination is to become more difficult, as the Chinese health authorities vowed on Wednesday to crack down on such practices that impacts gender balance.

The National Health and Family Planning Commission said in a statement that some agencies collected pregnant women's blood and sent it overseas to determine the gender of the baby, aggravating the nation's gender imbalance. The agents advertise on the Internet and will go to the customers' home or elsewhere to draw blood.

At the end of 2014, aggregate population in the Chinese mainland reached 1.36 billion, with a sex ratio of 105. The ratio for newborns stands at 116, much higher than the world average.

"China's sex imbalance problem among newborns is the most serious and prolonged in the world and the population it affects is the greatest," the commission said.

It acknowledged that the primary cause of sex imbalance, the Chinese preference for sons, has led to the abortion of many girls.

Fetal gender determination is banned in China, except in cases of "medical necessity", but the law does not ban abortions.

The commission said it would strengthen supervision of online advertisements on overseas fetal sex determination and ban search engines from linking to websites containing such ads. Illegal agencies on this "underground industrial chain" will be severely punished.

The commission also warned medical staff, reiterating a ban on carrying, mailing or transporting blood samples abroad.