BEIJING (AP) _ Police in eastern China have busted a human trafficking ring involving poor migrant couples who were selling their babies, a state-run newspaper reported Friday.

Police in Shandong province's Zoucheng city found last month that 17 infants had been sold in the city to Chinese buyers, the Global Times newspaper said. Police rescued 13 of the babies and sent them to welfare centers, and a search is under way for the other four, the paper said.

The report cited an investigating police officer as saying the couples were mainly migrants who had moved from poor areas in Sichuan province in southwest China to Zoucheng to seek work.

It quoted the officer, Chen Qingwei, as saying the husbands would go out to work while their wives sold their babies to raise money.

There was no immediate comment from police in Zoucheng.

One couple had sold three children, the newspaper said.

Chen said baby boys could be sold for up to 50,000 yuan ($7,730), while the price for girls was 30,000 yuan, much more than the parents could earn from farming.

There is a thriving black market in children in China _ mostly involving buyers who either want more children or want them as slave labor _ that endures despite harsh penalties for traffickers, including death. The country's one-child policy limits most urban couples to one child and rural families to two.

In July, authorities in southern China rescued 89 trafficked minors, including one as young as 10 days old, and arrested 369 suspects after uncovering two child trafficking gangs.