LAGOS (AFP) - Nigerian police on Wednesday said they had found six pregnant teenage girls in a raid on a house and arrested three people suspected of planning to sell their babies.

It was the second so-called baby factory uncovered in a week in the west African nation.

"We acted on intelligence information and raided the house in Enugu (city) where we met six girls, under 17 and all pregnant, and freed them," police spokesman Ebere Amaraizu in south-eastern Enugu state told Agence France-Presse.

He said two men and a woman believed to be operating a child-trafficking ring were arrested during the raid on Monday and were cooperating with police.

Mr Amaraizu said the girls had been "lured into the house with a promise of some money after" delivering a child.

"Investigation will unravel the details. We have to know how they came about the pregnancy and where they came from," he said.

Monday's raid came five days after police in nearby Imo State freed 17 pregnant girls and 11 small children from a home in the town of Umuaka.

The girls, aged between 14 and 17, told police that they had been impregnated by a 23-year-old man who is currently in custody. The owner of the building is on the run.

Nigerian police have uncovered a series of alleged baby factories in recent years, notably in the south-eastern part of the country, but the intended buyers of the children have not been established.

Mr Arinze Orakwue of the National Agency for the Prohibition of Traffic in Persons and other Related Matters (Naptip) said the black market for the newborns includes those who want to avoid the legal adoption system.

"It's cultural," he said, noting that couples in Nigeria who struggle to conceive often face social stigma and may not want to discuss their situation with an adoption agency.

"What is happening in those states is a dangerous development. It is simply a criminalisation of the child adoption law," he said.

He added that security agencies needed to do more to protect impoverished teenagers who may be persuaded to participate in such a scheme.

"It does not matter whether the girl willingly agrees to have the baby or she is forced to do so," he said. "It does not also matter whether she is paid or not for having the baby. It's criminal."

Human trafficking, including the selling of children, is the third most common crime in Nigeria behind fraud and drug trafficking, the United Nations has said.

In a human trafficking report released last month, the European Union identified Nigeria as the African country where the scourge is most common.

The report said the selling of children was widespread.

In May of 2011 in south-eastern Abia state, police freed 32 pregnant girls who said they had been offered to sell their babies for between 25,000 and 30,000 naira (S$198 and S$237), depending on the sex of the baby.

Another 17 pregnant girls were discovered in southern Anambra state in October 2011 under similar circumstances.

Nigeria is Africa's top oil producer, but poverty is endemic across the country and most of the estimated 160 million people still live on less than two dollars a day.