Users of social networking sites such as Facebook and Line are accustomed to receiving a "friend" request or a message from an unfamiliar contact. Most people ignore them, but the few who respond may need to think twice before pushing the send button.

Cases of "sextortion," in which victims are lured into sending explicit images of themselves and are then blackmailed by the recipients, have been reported to police and cybersecurity firms, prompting them to raise a red flag.

Cases of "sextortion," a term coined from the words "sex" and "extortion," have become more common in the U.S. and other countries such as the Philippines in the past few years. But a sextortion crime ring is now apparently targeting Japanese social media users as well, according to police and cybersecurity companies.