Images of child abuse posted and sold online are rapidly becoming more graphic and more sadistic and involving younger children, a British-based Internet monitoring group said on Tuesday.

In its annual report, the Internet Watch Foundation (IWF) said in the last year there had been a four-fold increase in the number of images involving severe abuse such as penetrative and sadistic sexual activity.

The group said nearly 60 percent of all commercial child abuse Web sites now sold images of child rape.

"Sadly we have to report new trends regarding the young age of the child victims in the images we assess and the dreadful severity of abuse they are suffering," said Peter Robbins, the IWF's chief executive.

The body, the official British organisation for the public and IT professionals to log suspicious content, said the number of reports of child pornography it had received in 2006 had risen by 34 percent compared to the previous year.

It found that 80 percent of those in the abusive images were female and 91 percent appeared to be under 12.

"This is babies, toddlers and pre-pubescent children suffering some of the most horrific abuse," an IWF spokeswoman told Reuters.

"Sadly the commercial sites are just responding to a demand by people around the world to buy that level of image. If there's profit to be made they will carry on doing it."

The vast majority of Web sites were linked to the United States or Russia, where they are often run by organised criminal gangs.

About 90 percent of all commercial sites appeared to be hosted in the two countries while more than 80 percent of the 3,077 Web sites that contained potentially illegal content were linked to Russia or the United States.

The IWF spokeswoman said although the overall number of Web sites had remained roughly the same, each one was putting out more content with a 74 percent rise in the number of individual URLs containing child abuse.

There had also been a significant rise in the abuse of photo-sharing Web sites. In 2004, the IWF database had no images that were posted on sites which allow users to post their holiday snaps for friends and family to see, she said.

Last year, such sites accounted for 10.5 percent of the URLs with child abuse content.

The IWF said its main challenge was trying to target the commercial sites which often "hopped" to different servers around the world.

It called for better international cooperation to ensure the content was removed, saying one offending site had been reported 224 times since 2002.

"As quickly as we can report them to the relevant law enforcement agencies, they are probably gone again and in a different country's borders," the IWF spokeswoman said.