Sick blackmailers are encouraging people to undress and perform sex acts on camera before threatening to post them online.
Detectives have today issued a warning about the crime - known as 'sextortion' - after there were four reported cases in recent weeks in Sheffield and Rotherham.
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Social media users have been contacted by blackmailers and persuaded to undress and perform sexually explicit acts in front of a web cam.
They then demand high sums of money - as much as £1000 in one case - from the victim otherwise they threaten to publish the footage online or send them to their friends and family.
None of the victims have parted with cash but police said those targeted had been left 'traumatised'.
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Officers believe the problem could be even more widespread with many more victims than the four reported cases so far.
Detectives have now warned internet users across South Yorkshire to be on their guard against the scam.
Detective Constable Greg Dakin said: “These are very serious cases and have caused a great deal of upset to the victims.
"None of the victims paid the money demanded, which in some cases was as high as £1000, but they have been left traumatised by the experience and we believe there are many more victims who choose not to report incidents to police.”
Facebook and WhatsApp were used to target the victims of the latest offences.
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DC Dakin added: “Typically there are two types of demand made of the victim, either for further indecent photographs of the victim to be forwarded, or a demand for money, which was the case in three of these four reported incidents.
“Consider very carefully about who you befriend online, especially if you are considering sharing anything intimate with them. Even if do you know them, could you share something with them that they could use against you in the future.
"Once you start using your webcam or send pictures over the internet, you will never have control over what happens with those videos or pictures. They may exist forever on the internet.”
Police issued the following advice if you think you may have become a victim:-
* Don’t panic. Contact police on 101
• We will take your case seriously, not judge you, and deal with your case in confidence
• Do not continue to communicate with the criminal
• If possible take photos of what is on the screen or screenshot the detail. Companies like Facebook and YouTube have online reporting systems. Google them and report the crime. Think about any other accounts or details you may have given the criminal
• Do not pay demands. While you might be tempted to, you have no guarantee they won’t ask for more. You also have no control over whether the video is circulated
• If you can, preserve as much evidence as possible.