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.

“We’ve received five calls asking us for advice since last December,” said Ayumi Shiraishi, an official at the government-affiliated Information-Technology Promotion Agency, which posted a warning about sextortion cases in Japan last December.

The National Police Agency says it does not keep track of the number of such cases. However, police in Chiba arrested a 43-year-old Japanese man from Fukuoka Prefecture in March 2014 for allegedly extorting money from a man in his 30s in the Tohoku region.

According to the police and anti-virus software-maker Trend Micro, which tracks cybercrime, a Chinese accomplice had met the victim while posing as a woman on a matchmaking site and the two started exchanging messages on Line.

After several exchanges, he asked to swap explicit videos, and the victim was told to install an Android app on his phone. However, the app collected and sent his contact information to the perpetrator, according to Trend Micro’s report, “Sextortion in the Far East,” which was released in March.

The Fukuoka man then allegedly called the victim and threatened to publish the explicit video if he did not hand over money. The victim ended up sending him ¥200,000.

According to police, the Fukuoka man said he had been hired to target Japanese by a crime syndicate that included Chinese and South Korean members.

They operated using manuals in various languages, the man was quoted as telling investigators.

Shiraishi said to avoid becoming a victim of sextortion, users need to keep in mind two points: “Only install apps from a trustworthy site and don’t send private images you don’t want to be seen, even to your boyfriends and girlfriends.”

Users with Android smartphones need to be especially careful because, unlike iPhone users, they can download apps from unauthorized makers, she said.

“Once you’ve sent (an image), you can’t get it back,” she warned. “You have to keep in mind that there is always the possibility that the person you’ve sent them to will spread it on the Internet.”