Some 50 million migrant children worldwide were living in other countries or were still living in their native countries after having been forcibly displaced from their homes in 2016, according to the United Nations Children's Fund.
The September 2016 report titled "Uprooted: The Growing Crisis for Refugee and Migrant Children," said "migrating and displaced children are at risk of some of the worst forms of abuse and harm."
The report said the 10 countries that hosted the largest number of refugees are in Asia and Africa. Turkey had by far the largest number of refugees.
More than 100 countries detain children for immigration reasons, according to the Global Campaign to End Child Immigration Detention.
The campaign has researched only a small number of countries that detain children on the basis of their immigration status: Australia, Malaysia, Israel, South Africa, Greece, Mexico and the United States.
The UNICEF report said some countries do not detain migrant children at all, while others separate them from their families by placing them in child care facilities. Other countries detain children with convicted adult felons.
There is, however, a lack of verified information on the number of children "deprived of liberty each year," said the United Nations Committee on the Rights of the Child and more than 50 other human rights groups.
While the campaign estimates that hundreds of thousands of children are locked in immigration centers every day, Human Rights Watch said in its report, "Children Behind Bars: The Global Overuse of Detention of Children," that more than 1 million children are behind bars around the world.
The Human Rights Watch report also cited a U.N. human rights treaty that prohibits the "mandatory and indefinite detention of children."
Nevertheless, the report said Australia has had a mandatory detention policy for all asylum seekers since 1992. At the end of October 2015, Australia held 112 children in mainland detention centers and another 95 children in a regional center on the island of Nauru.
In the U.S., the administration of President Barack Obama "dramatically expanded family immigration detention capacity" from 100 beds to more than 3,000 in an attempt to deter migrants from Central America, the report said.
The Obama administration, however, subsequently "backed away from that rationale."
The report said Thailand's immigration laws allowed the "indefinite detention of all refugees" and that migrant children were detained in "squalid cells without adequate food or opportunity to exercise or receive an education."
In Indonesia, Malaysia and Mexico, there is "large-scale detention of migrant children," according to Human Rights Watch and other organizations.
