More than 10,000 unaccompanied refugee and migrant children have disappeared in Europe, the EU police agency Europol said on Sunday, fearing many have been whisked into sex trafficking rings or the slave trade.

Europol's press office confirmed to Al Jazeera the figures published in British newspaper The Observer.

The number relates to the past 18-24 months.

Brian Donald, the agency's chief of staff, said that the vulnerable children had disappeared from the system after registering with state authorities after their arrival in Europe.

"It's not unreasonable to say that we're looking at 10,000-plus children," Donald told The Observer, adding that 5,000 had disappeared in Italy alone.

"Not all of them will be criminally exploited; some might have been passed on to family members. We just don't know where they are, what they're doing or whom they are with."

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Donald said there was evidence of a "criminal infrastructure" established since mid-2014 to exploit the refugee flow.

The Observer reported that Europol found evidence of links between smuggling rings bringing people into the European Union and human trafficking gangs exploiting migrants for sex and slavery.

"There are prisons in Germany and Hungary where the vast majority of people arrested and placed there are in relation to criminal activity surrounding the migrant crisis," Donald said.

More than one million migrants and refugees, many fleeing the Syria conflict, crossed into Europe last year.

"Whether they are registered or not, we're talking about 270,000 children," Donald told the paper.

"Not all of those are unaccompanied, but we also have evidence that a large proportion might be," he said, adding that the 10,000 is likely to be a conservative estimate.

He said many of the children are "visible", and not "spirited away and held in the middle of forests".

'Invisible to authorities'

Raffaela Milano, Save the Children's Italy-Europe programme director, said that "unaccompanied minors who travel without adults are the most vulnerable group of the migratory flow".

"Many minors, in fact, make themselves 'invisible' to the authorities to enable them to continue their journey in Europe, for fear of being sent back," she said.

Many children arrive first on the Greek islands before making the journey to relatives across Europe.

Laura Pappa, president of the Greek charity Meta-Action, a group accompanying children who travel without relatives, said that they "face a destiny that is worse than that of the rest of migrants waiting to be relocated".

She said that they often have to wait for around seven months to be reunited with relatives, and that procedures can be slow and complicated.

"There are some people that present themselves as uncles and take the children. It's not easy in this mess to cross check the identity of the 'uncle'."

Pappa said the group has helped 3,000 children reach family, but that it "is not enough".

Britain is one country that has said it will take in migrant or refugee children who have been separated from their parents.

Deaths continue

Despite the constant risk of death and deportation, refugees continue to stream into Europe, risking their lives to escape poverty, repression and conflict.

Many children are among the refugees and migrants who have lost their lives making the perilous crossing in the Mediterranean.

In the latest tragedy, the Turkish coastguard recovered the bodies of women and children were washed up on a beach after their boat sank, leaving at least 37 people dead.

READ MORE: Deadliest January on record for refugees raises alarm

Tensions are escalating across the continent over the increasing numbers of refugees, with many right-wing groups in Europe calling for more immigration restrictions and tighter borders.

On Saturday, Swedish police said dozens of masked men believed to belong to neo-Nazi gangs gathered in Stockholm and handed out leaflets calling for attacks against young unaccompanied migrants.

In England on Saturday, anti-fascist and far-right protesters, pro and anti welcoming refugees respectively, clashed in the coastal town of Dover.

Similar clashes erupted in a southern German town on Friday, where unknown assailants threw a hand grenade into a refugee shelter. More than one million asylum entered the country last year.