In the first attempt by law enforcement agencies to quantify one of the most worrying aspects of the migrant crisis, Europol’s chief of staff told The Guardian that thousands of vulnerable minors had vanished after registering with state authorities.

Brian Donald said 5,000 children had disappeared in Italy alone, while another 1,000 were unaccounted for in Sweden. He warned that a sophisticated pan-European “criminal infrastructure” was now targeting refugees.

“It’s not unreasonable to say that we’re looking at 10,000-plus children. 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.”

The plight of unaccompanied child refugees has emerged as one of the most pressing issues in the migrant crisis. Last week it was announced that Britain would accept more unaccompanied minors from Syria and other conflict zones.

According to Save the Children, an estimated 26,000 unaccompanied children entered Europe last year. Europol, which has a 900-strong force of intelligence analysts and police liaison officers, believes 27% of the million arrivals in Europe last year were minors.

“Whether they are registered or not, we’re talking about 270,000 children. Not all of those are unaccompanied, but we also have evidence that a large proportion might be,” said Donald, indicating that the 10,000 figure is likely to be a conservative estimate of the actual number of unaccompanied minors who have disappeared since entering Europe.

The police agency has also documented a disturbing crossover between organized gangs helping to smuggle refugees into the EU and human-trafficking gangs exploiting them for sex work and slavery. He said that longstanding criminal gangs known to be involved in human trafficking, whose identity had been logged in the agency’s Phoenix database, were now being caught exploiting refugees.

“The ones who have been active in human smuggling are now appearing in our files in relation to migrant smuggling,” said Donald.