Millions of children are growing up in squalid urban areas and denied basic services despite living close to them, the United Nations Children's Fund said Tuesday.

UNICEF said children living in slums and shantytowns often lack water, electricity and health care and it urged policy makers to ensure urban planning meets the needs of children.

The agency said it is common for statistics to show that, on average, children growing up in cities are better off than those in rural areas, which often leads to missing the plight of poor, urban children.

"This conceals the fact that the greatest inequities are found within towns and cities," it said. "In most urban areas, great opportunity and great deprivation exist side by side."

UNICEF said more than one-third of children in urban areas don't ever get birth certificates, which means they are invisible to authorities and can't get into social programs. This rises to half of all children in parts of Africa and Asia, two of the world's regions seeing the fastest migration from rural to urban areas.

More than half of the world's population lives in cities and towns, including a billion children. By 2050, two-thirds of the world's people are expected to live in urban areas, UNICEF said.

Many poor children in urban areas don't go to school and instead are forced to work to help their families, often in dangerous jobs. Tens of millions of children live or work on the streets and "the number is rising with global population growth, migration and increasing urbanization," the report said.

Children are also vulnerable to people traffickers. UNICEF said nearly 2.5 million people are in forced labor as a result of trafficking, and a quarter to half of them are children.

Nearly 8 million children died in 2010 before reaching the age of 5, about a third of them from hunger, the agency said. The rest died of pneumonia, diarrhea or birth complications.

In urban areas poor sanitation and overcrowding drive up child mortality, UNICEF said.

"When we think of poverty, the image that traditionally comes to mind is that of a child in a rural village," UNICEF director Anthony Lake said in a statement. "But today, an increasing number of children living in slums and shantytowns are among the most disadvantaged and vulnerable in the world, deprived of the most basic services and denied the right to thrive."