Facebook has said it has a zero tolerance approach to child exploitation on its site, despite claims it failed to remove sexualised images of children.
A BBC investigation uncovered secret groups on the social networking site which it said paedophiles are using to swap obscene images of children.
The broadcaster said it reported 20 images to Facebook.
Of these some were taken down by users, but only four were removed by the company, leaving half of them still up, the BBC said.
One image was described as showing a girl said to be aged 10 or 11 wearing a vest, accompanied by the caption "yum yum", while a number of innocent pictures of children stolen from other locations were accompanied by obscene posts.
Facebook said these examples did not breach community standards, the BBC said.
When asked about these incidents a spokeswoman for the social media giant told the Press Association it could not comment "because we have not been provided with any information on the groups in question".
It said it will investigate fully and remove groups "once we confirm they are against our rules".
Groups set up on Facebook can have one of three settings - public, closed or secret. In a secret group only those who are members of the group can see what content is posted, and only current or former members can search for the group.
An NSPCC spokesman called on Facebook to carry out an urgent review of what is being shared in such groups.
He said: "It beggars belief that paedophiles have been able to set up secret groups to share images of children and discuss their sick fantasies on the world's largest social network.
"We have said before that social media companies need to take more responsibility for the content on their sites. Facebook needs to urgently review what is being shared through these groups, use its technical expertise to identify anyone who could pose a threat to children's safety, and share this intelligence with law enforcement agencies."
A spokeswoman for Facebook said: "We have zero tolerance for child exploitation on Facebook. This illegal behaviour is rare on Facebook but it is immediately removed and reported to relevant law enforcement agencies when it is detected.
"We believe that our comprehensive reporting system and community standards help make Facebook one of the safest places for people on the web. We urge people on Facebook to use the reporting tools available on every page so we can investigate and take swift action."