The exploiters are increasingly cunning

Japanese Justice Minister Mayumi Moriyama

The internet has made tackling the problem much harder too, said Japanese Justice Minister Mayumi Moriyama in a keynote speech.

More than 3,000 delegates from 138 countries are attending the four-day conference in Yokohama, Japan - the second of its kind.

The World Congress against Commercial Sexual Exploitation of Children aims to chart progress since the first meeting five years ago, when delegates agreed to create national plans of action and build a database of offenders and children at risk.

Paedophiles are increasingly using the internet

"The exploiters are increasingly cunning, and most worryingly, there is a huge increase in paedophile sites on the internet," said Ms Moriyama.

And Japanese Foreign Minister Makiko Tanaka, chairing the event, said action needed to be taken across the world.

"Five years ago, we gathered in Stockholm to bring an end to these hateful criminal acts. Today, despite our efforts, the situation is getting worse," she said.

Governments have been working on strategies to tighten their laws against child prostitution, child pornography and the trafficking of minors.

Thriving trade

The BBC's Elizabeth Blunt, who attended the Stockholm conference, says one real achievement has been changes to the law in certain countries to allow abusers to be prosecuted at home for crimes committed against children abroad.

But the illicit trade is still flourishing worldwide, the UN Children's Fund (Unicef) said.

A Unicef report released last week said that about one million children worldwide are sold into sex every year.

There are more than 3,000 delegates

It called on every government to adopt and enforce laws against the sale of children, child prostitution and pornographic material involving children.

In the Philippines alone there are estimated to be about 100,000 child prostitutes - five times the number there were 15 years ago.

Unicef executive director Carol Bellamy, quoted by the Associated Press news agency, said that in South-East Asia and West Africa "governments are taking these issues more seriously because there has been more public attention".

But solutions would be difficult to find because the problems and numbers of children involved varied greatly from country to country, she said.

Unicef estimates that about one-third of sex workers in the Cambodian capital, Phnom Penh, are between 12 and 17 years old. And there are believed to be almost 500,000 under-age prostitutes in India alone.