Thousands of children unaccounted for since Haiti’s earthquake are at risk of falling prey to child traffickers, aid agencies have learned, as fears were raised over at least 15 children who have vanished from hospitals within the past few days.

Unicef, the U.N. children’s agency, warned that "traffickers fish in pools of vulnerability. We know from past experience that trafficking happens in the chaos that usually follows emergencies." A Unicef adviser, Jean Luc Legrand, said he knew of at least 15 cases of children disappearing from hospitals.

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Save the Children, World Vision and the British Red Cross have called for an immediate halt to adoptions of Haitian children not approved before the earthquake, warning that child traffickers could exploit the lack of regulation. There has been a surge in offers from well-meaning foreigners.

Rupert Colville, spokesman for the UN High Commissioner for Human Rights, said that child enslavement and trafficking was "an existing problem and could easily emerge as a serious issue over the coming weeks and months."

Nearly 30 agencies helped by the UN peacekeeping mission and the Haitian government are urgently pooling information and resources to counter the threat. They are are touring hospitals and orphanages, broadcasting radio messages, and increasing surveillance of road traffic, the airport and the border with the Dominican Republic.

The scale of the problem is potentially enormous. Haiti is awash with children, with 45 percent of its population younger than 15. One U.N. official estimated that between 40,000 and 60,000 children were killed, orphaned or separated from their families by the earthquake, which struck while most were still in school, and anecdotal evidence suggests many have been left to fend for themselves.

One small orphanage visited by The Times yesterday said it had turned away ten children because its buildings were badly damaged. A World Vision official in Jimani, a town just across the border in the Dominican Republic, said eight orphans and 25 unaccompanied children — many injured — had turned up there by Tuesday. A UN official spoke of people driving to the airport in expensive cars and putting children on outgoing flights without any documentation.

The alarm is particularly acute given Haiti’s dire record of child abuse. The UN High Commissioner for Refugees reported in 2008 that 29 per cent of children under 14 were already working, and roughly 300,000 were ‘restaveks’ (a creole corruption of ‘rester avec’) whose impoverished parents send them to work for wealthier families in the hope they will receive food and shelter.

Some were cared for and educated, but others were "sexually exploited and physically abused; and are unpaid, undocumented, and unprotected". When they turn 15, and must by law be paid, many are turned on to the streets to join as many as 3,000 other children who survive on the streets of Port-au-Prince as vendors, beggars or prostitutes.

Even before the earthquake, Haitian children were regularly sent to the Dominican Republic to work in sex tourism, or recruited by armed gangs. A Haitian women’s organisation documented 140 rapes of girls younger than 18 years in the 18 months to June 2008. Haiti’s many orphanages — there are said to be 200 in Port-au-Prince alone — are poorly regulated, and some are mere fronts for international child traffickers.