Police in eastern China have broken up a human trafficking gang that bought babies from poor families and sold them on for as much as $8,000, state media said Friday.

Authorities in Shandong province last month detained 15 members of the gang who had paid women from other parts of China to bear children which they then sold to others, including couples unable to conceive and those wanting sons.

In a microblog posting, police in Zoucheng city -- where the trafficking ring was uncovered -- said boys were sold for up to 50,000 yuan ($8,000) while girls could fetch up to 30,000 yuan.

The state-run Global Times newspaper said authorities had tracked down 13 children but were still searching for four other missing infants.

"Working as migrant workers here, the families mainly came from poverty-stricken areas. Husbands went out to work and wives sold their babies to raise money," police investigator Chen Qingwei was quoted as saying.

Police could not immediately be reached for comment on Friday.

Childless couples in China are currently allowed to adopt children from any source, which has led to a thriving underground child trafficking market.

Many academics blame the problem on the nation's strict "one-child" policy, which has also put a premium on baby boys, as many families want a male heir.

Abductions and trafficking cases have sparked public concern in China following a string of scandals.

In 2007, authorities found that thousands of people had been forced into slave labour in brickyards and mines across the nation, in a scandal that shocked the nation.

More recently, police said in July that they had freed 89 children in a crackdown on trafficking launched this year after online reports of widespread abductions sparked public outrage.

Police also arrested 369 people in the operation, which aimed to break up a pair of "large criminal enterprises" involved in child-trafficking across 14 provinces, they said at the time.