More than 10,000 asylum seeker children have vanished in Europe.

Thousands of vulnerable refugee children have disappeared in Europe, with authorities fearing they may have been taken by sex traffickers or absorbed into the sex trade.

According to the EU police agency Europol, more than 10,000 unaccompanied minors are now unaccounted for after registering with the state.

At least 5,000 have vanished from Italy alone, along with 1,000 more in Sweden.

“It’s not unreasonable to say that we’re looking at 10,000-plus children,” Europol’s chief of staff, Brian Donald, told the UK Observer.

“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 that a “criminal infrastructure” had been set up specifically to target asylum-seekers.

He also said that links had been found between the smugglers bringing people into Europe and organised trafficking gangs, which exploit 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,” he explained.

Around 270,000 children were among the million-plus Syrian refugees who entered Europe last year, with a significant proportion separated from their families.

The UN children’s agency UNICEF has urged European governments to protect them.

“We urgently call for a plan within Europe for unaccompanied and separated children covering family reunification, relocation and other alternatives so that children do not end up being abused and exploited by smugglers and traffickers,” it said in a statement.

Notably, the UK government refused a recent call to take 3,000 of the children, saying they would instead bring others directly from Syria and various conflict zones.

Mr Donald said the missing children are likely to be “hiding in plain sight” and called on the community to be vigilant.

“These kids are in the community, if they’re being abused, it’s in the community,” he said.

“They’re not being spirited away and held in the middle of forests, though I suspect some might be, they’re in the community — they’re visible. As a population we need to be alert to this.”