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More than 1,000 children and young people in Denmark have been charged with redistributing child pornography after sex videos featuring 15-year-olds were shared on Facebook Messenger.

Monday's move came after the California-based social media giant alerted U.S. authorities that the videos, along with a sexually explicit photo, were being spread online.

It also comes amid growing concern about sexting among school-age children and instances of revenge porn.

"It may be a prank, but it can have adult consequences"

“We have taken the case very seriously as it has major implications for those involved when such material is spread,” Inspector Lau Thygesen from Denmark’s North Zealand Police said in a statement. “And it must be stopped.”

Prosecutors will assess each case, Thygesen said, with punishments ranging from a criminal record entry to having their name added to child protection registers for 10 years.

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The 1,004 alleged offenders range in age from 15 to their early 20s; most shared the videos once or twice but some did so hundreds of times.

Denmark’s National Cyber Crime Center said the case began last year when Facebook received reports that the sex videos were being shared on its Messenger chat platform. It deleted the material and alerted U.S. authorities, who in turn contacted Europol and Danish police.

Authorities would not give details but said the victims were aged 15 when they were recorded. Although having sex at 15 is legal in Denmark, distributing images of anyone aged under 18 is a criminal offense. “This could be a violation of section 235 of the criminal code, distribution of child pornography,” the police statement said.

“It’s quite awful for those who appear on the videos, as their sexual act is now being shared online so many can see it,” said Police Superintendent Flemming Kjaerside of the National Cyber Crime Center told Danish television. “It's catastrophic for them and it can ruin their lives. The young people who shared this video may also be risking something quite ugly for their future … it may be a prank, but it can have adult consequences, since you can be convicted for distributing child pornography.”

He added that an entry on Denmark’s child protection register would prevent offenders from working in elementary schools or as sports coaches. “If U.S. authorities get information … it can also cause problems with entry into the United States, so it's serious and it has serious consequences far into the future,” Kjaerside said.

Related: Is social media contributing to rising teen suicide rate?

Alleged offenders were notified by email over the weekend; those aged under 18 were notified through their parents, police said.

"The interrogations start today,” National Cyber Crime Center Chief Superintendent Claus Birkelyng told Danish television Monday. "Many of the young people have already been summoned last week, so some of them have been made aware of it over the weekend and the young people will meet with the police today and in the coming weeks."

Facebook Messenger displayed on the screen of a smartphone. Dominic Lipinski / PA Wire file

Facebook said it took a "zero-tolerance approach to both non-consensual sharing of intimate images and child exploitation images."

A spokesman added: “Our systems run in the background and automatically remove and report intimate content involving children to the National Centre for Missing and Exploited Children, who in turn will review and dispatch the relevant information to the right law enforcement agencies across the globe."

Almost one quarter of U.S. seventh-graders send sexually suggestive texts or photos, with those sending explicit pictures especially likely to engage in sexual behavior, according to a 2014 study published in the journal Pediatrics.