Co-ordinated international efforts targeting fewer than 3,000 websites could effectively tackle the bulk of online child pornography, according to the Internet Watch Foundation (IWF).

The industry-funded watchdog recently carried out an in-depth study on the scale of the issue.

Concerted international action to disrupt persistent top level domains would block access to hundreds of thousands of online images depicting child abuse, the IWF believes.

The figure of 3,000 websites represents a "concrete target" and forms the basis for an international campaign.

"This year we have highlighted what we believe is a manageable number worldwide of such websites known to us," said IWF chief executive Peter Robbins.

"We hope that this revelation, and the analysis and intelligence behind the numbers, will lead to a better understanding of the issue and justify the need for more international partnerships to pool resources and thinking in order to find solutions."

The study showed that the number of child porn domains fell by almost 10 per cent between 2006 and 2007 thanks in large part to the internet industry's efforts to police itself.

However, despite the decline in the reported number of child porn websites, the increasingly extreme nature of online content is a cause for concern.

The IWF said that one in 10 images appears to involve children under the age of two, and a further one in three appears to depict children between the ages of three and six.

The organisation, which looked at English language only websites, found that the majority of child abuse sites are based in the US and Russia. Just a tiny minority can be traced to the UK.