The move follows the seizure of computer disks at a flat in Zandvoort, near Amsterdam, containing thousands of pictures of children, some as young as one, being abused.

An anti-paedophile group in neighbouring Belgium, Morkhoven, says it holds many more documents belonging to the owner of the flat, who was shot dead in Italy last month.

The group, which is planning to hand over its documents to the Dutch authorities next week says it has evidence the Internet porn ring extends into the United States and Eastern Europe and is linked to a child smuggling network.

The extent of child pornography on the Net is an unknown quantity

Separate ring in Belgium

Police in Belgium meanwhile are investigating a separate paedophile ring.

A senior Belgian officer, Marcel Van Lacke, said at least five adults, two of them Dutch, were suspected of involvement in the ring.

Police have distributed the pictures of 340 children seen on pornographic videos seized at the end of last year in the Belgian town of Temse.

They hope this will help establish whether any of the children are registered as missing in Belgium.

The man in whose house the videos were found has been released after three months of questioning and the thrust of the investigation has now moved to Portugal where the cassettes originated.

There have been a series of child abuse scandals in Belgium in the past two years which have led to public protests and reforms of the police and justice system.