LONDON — Hundreds of millions of children suffer exploitation and discrimination but are virtually invisible to the eyes of the world, according to a UNICEF report published this week.

Millions of children who are trafficked — often for sex — or forced to work in domestic servitude "disappear" from sight every year into underground economies, the U.N. Children's Fund report said.

Other children, including those who live on the street, remain in plain sight but are denied fundamental services and protections like schooling and health care.

The report, "The State of the World's Children 2006: Excluded and Invisible," said the issue of exploited children often is missing from public debate, statistics and news stories.

Releasing the report in London, UNICEF Executive Director Ann M. Veneman said governments must strive to reach vulnerable children.

"To ensure that children are protected, the abuse and exploitation of children must be brought to light and those who violate children brought to justice," she said.

The report says children at risk include:

Children without a formal identity. Over half of all births in the developing world — excluding China — go unregistered, denying more than 50 million children recognition as citizens. Without a registered identity, children are not guaranteed an education, good health care and other basic services.

Children without parental care. An estimated 143 million children in the developing world have suffered the death of at least one parent. Around the world, tens of millions of children spend a large portion of their lives on the streets and more than 1 million live in detention, most awaiting trial for minor offenses.

Children in adult roles. Hundreds of thousands of children are caught up in armed conflict as combatants, messengers, porters, cooks and sex slaves. An estimated 171 million children work in hazardous conditions, including in factories, mines and agriculture.

Children who are exploited. Some 8.4 million children work in the worst forms of child labor, including prostitution and debt bondage. Nearly 2 million children are employed in the commercial sex trade. A large but unknown number of children are exploited as domestic servants in private homes.

The report said governments need to improve research and monitoring of children's needs, introduce better protective legislation and provide better funding for programs that help children. Many programs already in place also need reform to make them more accessible, it said.

Instability and insufficient health care are the main factors in high child mortality in some countries in the Middle East and North Africa, the report said.

Around 18 million children in the region are not registered at birth. In Sudan, the report said, just over half the children are registered.

It said working and street children often are forgotten and excluded from social services. Handicapped and HIV-positive children "are often kept at home and are unable to go to school or live normal lives."