Chinese police rescued more than 24,000 abducted children and women last year, some of whom had been sold for adoption or forced into prostitution as far away as Angola, officials said Sunday.

Trafficking of women and children is a serious problem in China -- blamed in part on the strict "one-child" policy, which has put a premium on baby boys -- and activists say the cases uncovered by police are just the tip of the iceberg.

The Ministry of Public Security said in a report posted on its website that police across the country had rescued 8,660 children and 15,458 women in 2011 -- all victims of trafficking.

The ministry added that authorities had cracked nearly 3,200 trafficking gangs last year, including a ring that sent Chinese women to Angola and forced them into prostitution.

"In November 2011, the public security ministry dispatched a police team to Angola... and detained 16 suspects and freed 19 Chinese women," it said.

Police also discovered that more than 2,000 children had been abducted and sold for adoption in 2011 -- a big problem in China where couples unable to conceive or wanting a son, or male heir, can adopt from any source.

In November last year, for instance, police in the eastern province of Shandong broke up a human trafficking gang that bought babies from poor families and sold them on for as much as $8,000.

Abductions and trafficking in China have caused huge public concern, but despite regular government vows to crack down hard on the crime, incidents still emerge on a regular basis.

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

More recently, police rescued 16 under-age girls forced into prostitution in the northern region of Inner Mongolia last April, and freed 89 children in July in a crackdown on trafficking launched earlier in the year.

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.

Nearly 11,300 people accused of trafficking were punished from 2008 to 2011 and the number of traffickers has shrunk, the official Xinhua news agency said earlier this month, without saying how they were punished.

The report also said there was a drop in the number of cases involving kidnapped babies, but added that a rising number of people were selling their own babies to earn money.

Most trafficked babies still go to desperate couples willing to illegally adopt a child, it said.

The ministry said it had built a nationwide system last year that collects DNA data from the parents of children suspected of having been kidnapped.

Police then input the DNA of children rescued from trafficking rings into the system, and if there is a match, can return the youngsters to their parents as quickly as possible.

The database also helps the police in cases of illegal adoptions, as they can prove that a child has been unlawfully taken in by a family if his or her DNA matches that of somebody reported missing.