Millions of the world's poorest children have fallen through holes in social safety nets because there is no official record of their birth, UNICEF said Wednesday.

In its annual 'State of the World's Children' report, the United Nations' Children's Fund said one third of the estimated 150 million children born each year were not registered and were therefore 'invisible' to organizations that are supposed to protect them from abuse and exploitation.

The agency stressed the need to step up efforts to reach these children, who are among the world's neediest and most vulnerable.

Nigel Fisher, CEO of Toronto-based UNICEF Canada, said invisible children are excluded from school, health care and their right to have a childhood.

"It's the kids that don't come to everybody's attention," he told CTV Newsnet Wednesday.

"We are working with governments to try and make registration mandatory, but a lot of the time it's about political will and resources."

The UNICEF report, entitled 'Excluded and Invisible,' focuses on children who 'disappear' when they become victims of human trafficking or are forced to work behind closed doors in domestic service.

Other so-called invisible children include homeless street kids and orphans.

In most cases, there is no official record of their presence in society, making it easy for governments to ignore them and for traffickers to dispose of them without risk of retribution.

The absence of official birth records for these children led to range of problems, including pedophile abuse and slavery, UNICEF said.

The report estimated that 1.8 million children end up in the sex industry, 5.7 million are sold into slavery and 1.2 million are trafficked each year.

AIDS orphans and those forced into early marriages also accounted for millions of children who simply disappeared either through being cast out by their communities and taking to the streets or just ceasing to be seen.