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BERLIN — Teenagers are being turned into "weapons" by ISIS, triggering fears that they could slip back to the West to launch attacks.

The head of the Germany's domestic intelligence service said authorities there had identified children aged as young as 13 who have left for ISIS territory or who are "prepared to stage attacks" in the country.

Some have been brought to Iraq and Syria by their parents where they have been brainwashed while others are being radicalized online, Hans-Georg Maassen told NBC News.

“When you know how ISIS treats children ... then you have to assume that you are dealing with people who are willing to unconditionally torture and kill for ISIS,” said Maassen, who is in charge of Germany’s Office for the Protection of the Constitution (BfV).

In February, a 15-year-old Muslim girl was arrested for attacking a German police officer with a knife at Hanover train station. She had been indoctrinated by ISIS' online propaganda.

However, security officials in Germany are limited in their actions against minors. The legal system prohibits surveillance of people aged under 16 and children are held "non-accountable" until the age of 14.

Child soldiers in the early stages of training with ISIS. Flashpoint Partners

Maassen called for an expansion of powers and resources for intelligence services in Germany. Some of the measures proposed by officials include tighter surveillance of convicted Islamists and the introduction of electronic tagging.

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“The attacks in Paris and Brussels were somewhat of a wake-up call,” Maassen said.

“Europe cannot think like it did in the 1980s and 90s" about counter-terrorism measures as the militants' tactics have evolved, the domestic intelligence chief added.

He believes that allowing broader surveillance measures could potentially help to prevent future attacks.

Stephan Kramer, from the local intelligence service in Thuringia told Mitteldeutsche Zeitung, also called for an adjustment to anti-terror laws because “minors are not only radicalized, they are also used as weapons."

He added: “We have a massive problem."

In addition to the potential threat of child militants, Maassen's agency has identified more than 8,600 radicalized Muslims and more than 450 violent extremists within Germany's borders.

Out of some 810 German jihadis estimated to have joined ISIS ranks in Syria and Iraq, about 260 trained militants have apparently returned to Germany and "pose a security challenge," Maassen told NBC News. He would not comment on how many children were thought to be radicalized, but BfV later confirmed about 40 minors, half of them females, had travelled to the conflict-torn nations.

Rob Bertholee, the head of Dutch intelligence, told a conference in Berlin earlier this month that his agency had identified a total of 70 minors from the Netherlands who had traveled to Syria and Iraq. He added that children aged as young as 9 were being recruited by ISIS for combat training.

German intelligence chief Hans-Georg Maassen and his Dutch counterpart Rob Bertholee at a security conference in Berlin earlier this month. Andy Eckardt / NBC News

Elmar Thevessen, a terrorism expert with NBC's German partner ZDF, said that German authorities were "fighting an uphill battle" with "self-imposed moral restrictions" tying their hands.

He added: "The legal restraints imposed by the Constitutional Court make surveillance of suspects more difficult."

German officials have repeatedly stressed that the threat of an attack in Germany or elsewhere in Europe remains “very high."

Related: ISIS Is Training Children to Be Terrorists

Maassen said that his agency receives up to four tip-offs a day about terrorist plots.

According to Germany's Federal Criminal Police (BKA), a total of 40 investigations are currently examining the background of migrants and refugees who are suspected of possible membership in terrorist organizations. Last year, Germany took in more than one million migrants, the largest influx since the end of World War II.

“I’m not telling a secret if I say that I’m worried about the high number of migrants whose identity we don’t know for sure because they entered without valid passports,” Maassen told the Berlin conference earlier this month.

Concerns among security officials grew after it was determined that at least two of the Paris attackers arrived in Europe disguised as migrants.

In order to tackle these new challenges, Maassen has repeatedly called for closer cooperation and enhanced exchange of information among security services at the international level.

In particular, intelligence provided by U.S. agencies has become vital for the European services and has helped to foil attacks in Germany.

“Americans and the Europeans know that they depend on each other,” Maassen told NBC News.

But recent revelations that the NSA spied on German officials, including Chancellor Angela Merkel, and two alleged cases in which German government workers spied for the CIA “considerably damaged trust,” he added.