About 150 dangerous Islamists are being held in prisons across Germany, according to figures from the Federal Criminal Police Office (BKA) published by German daily Die Welt on Wednesday.

The men are either serving jail sentences or are in custody on terror-related charges, the paper cited the BKA as saying.

The newspaper said there were also several "relevant persons" being held, which it said were those regarded as sympathizers or supporters of radical Islam.

"In the next few years we must expect a wave of extremists in our prisons," the German state of Hesse's Minister of Justice, Eva Kühne-Hörmann (CDU), told the newspaper.

Read more: German authorities target Islamist women's network

She referred to hundreds of investigations against Islamists, which are currently being conducted nationwide, many of them against jihadists returning to Germany from the Middle East after fighting with the "Islamic State" militant group.

German police opened more than 1,200 terror-related investigations in 2017, a fivefold increase on the previous year

Prisons struggles to cope

Kühne-Hörmann warned that the sheer number of Islamists in German jails "poses great challenges for our deradicalization and prevention work."

She said if prison was not used to help extremists reintegrate into society, there was a danger of "releasing radical Islamists and those without prospects back into German society."

In December, German authorities admitted they had opened five times as many terror investigations in 2017 compared to the year before. Some 1,200 probes involved suspected acts of terrorism, and more than 80 percent of them had an Islamist connection.

Read more: Number of Salafists in Germany reaches record high

They included radical preachers, failed assassins and those returning from fighting in Syria and Iraq.

Die Welt cited Germany's prison officers' association as saying the number of Islamist inmates posed an increased security risk for staff and calling for additional training.

In neighboring France, prison guards went on strike last month, complaining about violence in the country's overcrowded prisons.

Prison officers had walked off the job after a jailed Islamist militant from Germany slashed at prison guards with a pair of scissors.

mm/aw (dpa, KNA)

DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.