Over 10,000 unaccompanied asylum seeker children have disappeared in Europe, the EU police agency Europol says, fearing many have been whisked away into sex trafficking rings or the slave trade.

Key points: Europol says over 10,000 asylum seeker children have gone missing

Europol says over 10,000 asylum seeker children have gone missing It is believed many may have fallen into sex trafficking and slavery

It is believed many may have fallen into sex trafficking and slavery Many children ditch their families in hope of improving their chances at asylum

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

"It's not unreasonable to say that we're looking at 10,000-plus children," Mr Donald said, 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."

Mr Donald said there was evidence of a "criminal infrastructure" established over the last 18 months to exploit the asylum seeker flow.

UK paper The Observer reported Europol found evidence of links between smuggling rings bringing people into the EU 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," Mr Donald said.

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

Europol has estimated that 27 per cent of them are children, The Observer said.

"Whether they are registered or not, we're talking about 270,000 children," Mr 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.

Unaccompanied minors the most vulnerable group

Raffaela Milano, Save the Children's Italy-Europe program director, said "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.

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

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

Many children are among those 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 who washed up on a beach after their boat sank, leaving at least 37 people dead.

Tensions are escalating across the continent over the increasing numbers of asylum seekers, with many right-wing groups 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.

AFP