THE HAGUE — 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 to 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 nongovernmental 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.’’

In a separate development Monday, French officials demolished a makeshift church and mosque as part of a security operation at a camp in Calais with thousands of migrants.

Authorities are trying to reestablish order in the long-lawless camp, and Monday’s operation was the culmination of a two-week effort to clear a 100-yard security zone around the perimeter of the camp, a regional official said.

The migrants and charity groups helping them were informed Jan. 19 of the pending demolition, the official said, adding that no one was hurt. However, the pastor of the church said authorities had told him the church was ‘‘safe’’ from destruction.

Officials in Sweden, a popular destination for migrants, said they were already aware of the missing children 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 are 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 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.

Last week Britain said it would accept an unspecified number of refugee children, after charities and other advocates 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.

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

Of the 150,000 migrants and refugees rescued at sea and brought to Italy in 2015, accompanied minors accounted for 12,360, the UN has said.