THE HAGUE, Netherlands (AP) — Authorities dealing with Europe's migrant crisis have lost track of about 10,000 unaccompanied children amid fears that organized crime gangs are beginning to exploit the vulnerable youngsters, a senior official at the European Union's police agency said Monday.

Europol Chief of Staff Brian Donald said that the figure "would be a conservative estimate across all the countries that are dealing with this migrant crisis" over the past 12-18 months.

The revelation that so many youngsters are unaccounted for is the latest worrying development in the migrant crisis and underscores the risks faced by people fleeing conflict, poverty and persecution in the Middle East, Africa and Asia even once they have reached the apparent safety of Europe.

Donald said the estimate of 10,000 missing was based on reports by law enforcement authorities, governments and non-governmental organizations.

"They're lost in the system," he said of the minors. "I think our concern is that we know that there are people out there who will exploit minors. We know there are people who will take them and use them for their own purposes."

Sweden, a popular destination for migrants, already is aware of the problem.

The Stockholm county government released a report last week citing Swedish Migration Agency statistics that said 1,900 of the 55,000 unaccompanied minors who have applied for asylum in Sweden in the past six years disappeared. The whereabouts of 1,250 of those is still unclear. About 88 percent of those who went missing are boys.

"There is very little information about what happens after they disappear. These children are particularly vulnerable to being exploited in various ways," the report said.

Amir Hashemi-Nik of the Stockholm County Administration said some of those who disappear are believed to be in the grip of human-trafficking rings and end up in prostitution, begging or other criminal activities. some disappear simply because they don't like the place where they have been assigned and decide to leave, others go when they approach their 18th birthday because they are worried it will be harder to get asylum.

North African boys are particularly likely to go underground because, unlike Syrians or Afghans, they are unlikely to get asylum.

"Many of them have lived on the streets in many other countries before coming to Sweden," the report said. "Many of these children become involved in crime."

Last week Britain announced it would accept an unspecified number of refugee children, after charities and opposition politicians pressured the government to help the thousands of unaccompanied minors fleeing conflicts in Syria and elsewhere.

The charity Save the Children has urged Britain to accept 3,000 children immediately. It estimates 26,000 minors arrived in Europe last year without adults, and are at risk from traffickers and sex abusers.

The British government, however, says it will take children from refugee camps in the Middle East rather than those already in Europe. It also will provide more funding to the European Asylum Support Office to help Greece and Italy reunite migrants, including children, with family members already in Europe

Europol analysts studying law enforcement details from across the 28-nation EU are concerned that they are beginning to see cross-pollination between people-smugglers and criminals who traffick and exploit humans.

"That confirms our understanding of criminal organizations at the European level," Donald said. "They are very adept at making changes to reflect the current situation. So if the market for them is changing then they will follow that market and at the moment the area of exploitation that's largely available is the exploitation of migrants."

Of the 150,000 migrants and refugees rescued at sea and brought to Italy in 2015, accompanied minors accounted for 12,360, according to the UNHCR office in Rome.

UNHCR official spokeswoman Carlotta Sami said there is no firm figure on just how many of those have slipped away or might have been exploited.

"More than disappeared, they are on the move," often trying to reach relatives or other contacts in northern Europe, not wanting to stay in Italy, Sami said. "There is no certainty where they are" or if they ever reached their destination.

Save the Children Italy spokesman Michele Prosperi said that Europol, "sounded an alarm that is based on a real risk" because thousands of minors don't stay within the system of migrant protection.

"They find themselves in a very vulnerable condition, and can be subject to violence or pressure," Prosperi said. They look at the traffickers not as a threat but as people who are trying to help them finish their journey, he added, and are very reluctant to tell anything about their treatment to aid workers or officials in Italy for fear that their plans to continue onward will be jeopardized.

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Associated Press writers Karl Ritter in Stockholm, Sweden, Jill Lawless in London, Frances D'Emilio in Rome and Colleen Barry in Milan contributed to this report.