The victims are usually teenage girls

The transfers are made using "even more cruel and devious means than the original slave trade," Unicef's Kul Gautum told an International Symposium on Trafficking of Children, being held in Tokyo.

He said in Asia and the Pacific alone, more than 30 million children have been traded over the last three decades.

A combination of poverty, globalisation, organised crime and discrimination against women encouraged the trade.

The victims are usually teenage girls who end up working in sweat shops or brothels, he said.

But ending the trade in humans is virtually impossible given the level of corruption among government officials, Mr Gautum said.

Police protection

"In some countries, police, who are supposed to stop these crimes, are involved in crimes by offering protection to criminals. Pimps and middlemen get protection from the police."

Mr Gautum said officials needed to be trained and made more accountable.

Japanese parliamentary secretary for foreign affairs Shinako Tsuchiya urged more co-operation between non-governmental organisations (NGOs) working in the field.

"There are many NGOs in the nations that ship our children, in places that are used as transfer points... but the truth is these NGOs' efforts lack co-ordination."

Educating women and children who run a high risk of being trafficked was also cited as crucial in preventing the trade.

In Bangladesh, Unicef is training 600,000 people to teach their peers about child trafficking.

More than 100 delegates attended the conference sponsored by Unicef and the Japanese foreign ministry.