Almost all the Vietnamese children who arrived in Dutch asylum centers in recent years disappeared and were probably trafficked, the Dutch government advisor on human trafficking has said.

Eighty unaccompanied minors from Vietnam went missing from government asylum centers in the Netherlands between 2015 and 2019, according to an investigation by the Dutch National Referral Site for Human Trafficking released this week.

That represents 97% of the total arrivals from Vietnam. Most are thought to have travelled on to Britain, where a study last year found that Vietnamese children were being made to work in nail salons, cannabis cultivation or prostitution.

"This confirms assumptions that a large number of these children end up in the hands of human traffickers," said Herman Bolhaar, the government's independent advisor on trafficking, in a statement on Wednesday.

He said it was "incomprehensible" that authorities had still not put regulations in place to combat this.

The Netherlands has between 5,000 and 7,500 victims of human trafficking a year, according to government figures. About one in five are victims of cross-border sex trafficking and 20 percent of those are minors.

Vietnam is consistently one of the top source countries for modern slaves in Britain - at least 3,187 suspected Vietnamese victims have been identified since 2009, official data shows.

But many come through other European countries and anti-slavery charities have said there is a lack of cooperation between different authorities to tackle the trend.

Ankie Broekers-Knol, Dutch minister for justice and security, said in a letter to parliament it was difficult for the Netherlands to achieve anything single-handedly.

"International cooperation is crucial in order to actually tackle this problem," she said.

But Shamir Ceuleers, spokesman for CKM, the Dutch Centre against human trafficking, said there was not enough urgency in the government's attitude to the problem.

"These are minors we are talking about," he told the Thomson Reuters Foundation.

"If Dutch children went missing we would also act. We want to see public prosecution and police becoming stricter on this issue, additional rules at the asylum centers, and the launch of a Europe-wide investigation."