Dutch authorities have issued an alert over the disappearances of dozens of pregnant African women housed in asylum shelters in the Netherlands.

The unusual alert, seen by the Guardian and Argos Radio of the Netherlands, was put out by the Expertise Centre for Human Trafficking and Smuggling (EMM), a collaboration between the Dutch National Police, the Royal Netherlands Military Police, the Social Affairs and Employment Inspectorate and the Immigration Service.

The EMM says it has knowledge of at least 25 heavily pregnant African women who vanished from asylum shelters in November and December. The majority of the women and girls are from Nigeria, but women from Cameroon, the Democratic Republic of Congo and Uganda are also among those missing.

Further investigation by the Guardian and Argos has revealed that women, including unaccompanied minors, pregnant women and those with newborn babies, continue to disappear on a weekly basis from asylum shelters across the Netherlands.

An internal EMM memo seen by the Guardian shows Dutch authorities believe it unlikely that these women would have chosen to leave the asylum shelters voluntarily. “The [women] disappear before they give birth or their asylum status is completed. This is not logical because they are legally resident here [in the Netherlands] and have a residence address, and access to all medical and maternity care that is required,” reads one section of the text.

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The memo highlights the serious concerns of Dutch authorities over the fate of these women: “It is not inconceivable that both the women and the newborn babies will become victims of human traffickers, illegal adoption or voodoo rituals.”

Thousands of Nigerian women and girls have been trafficked into Europe since the 2015 migrant crisis. Most are lured from Edo state with the promise of work, and travel through Libya before embarking on perilous boat journeys to Italy. Many then make their way to the Netherlands and the UK, where Nigerians remain one of the biggest groups trafficked from Libya. Although the trafficking of Nigerian women and girls into Europe is not new, the scale of the problem is: numbers have doubled since the migrant crisis.

A spokesperson for the Dutch organisation responsible for the housing of asylum seekers, the Central Agency for the Reception of Asylum Seekers (COA), said that every shelter has staff trained to recognise the signs of trafficking. He also emphasised that asylum shelters are not closed and that residents can come and go.

But the Guardian has seen internal notes from COA staff expressing fears about women in the shelters. These notes show that most women in the shelters who have applied for asylum have done so under a specific Dutch immigration category for victims of trafficking. Some of the files contain descriptions of forced sex work and other exploitation experienced in Italy and France, as well as accounts from women who say they are in debt to traffickers and are therefore under threat.

Ina Hut, director of CoMensha, the Dutch Coordination centre against human trafficking said:

“We find it very disturbing that these pregnant women disappear from the shelters, while they are entitled to all medical care and maternity care here. It is possible that they have fallen victim to traffickers, who put them under pressure and hold on to them via voodoo or threats of violence. This has to be investigated. We must do everything we can to protect this vulnerable group and to find out where these women have gone.”

Herman Bolhaar, the Dutch national rapporteur on trafficking, said: “The disappearance of pregnant women from reception centres in the Netherlands is a cause for great concern. We cannot be reassured without exactly knowing what happened to these women and their babies … It is appropriate and also necessary that the EMM is investigating this matter.”

Last year an investigation by the Observer and Argos revealed that over 60 Vietnamese children had disappeared from Dutch shelters to be trafficked into Britain. Dutch police suspect many end up working on cannabis farms and in nail salons.

“Last year we also raised questions about similar disappearances of Vietnamese children from Dutch asylum shelters. We owe it to these vulnerable groups to keep watch for signs of human trafficking. This is an essential part of the state’s responsibility to protect vulnerable people, on the national as well as the international level,” said Bolhaar.

Following this investigation, the Dutch parliament began an independent inquiry, currently been conducted by the EMM. The findings are due to be released within the next four weeks.

Sanne Terlingen, Ismail Einashe and David Davidson are members of the Lost in Europe cross-border journalism project, which investigates the disappearance of child migrants in Europe