At least 100,000 private Snapchat photos are believed to have been accessed by hackers – using a third-party service that saves images – who are threatening to release them onto the web.

The hackers said they had been intercepting and storing private photos sent via a third-party Snapchat client for years, and were now preparing to release them online onto a searchable database.

The images include naked photos and are likely to include images of under-18s, given Snapchat’s user base: half are aged between 13 and 17.

The mass hack, which is being called the Snappening, comes just weeks after hundreds of naked celebrity photos were leaked after an iCloud hack.

Posting on the website 4Chan, the hackers had reportedly been hinting about a leak for weeks, and announced it late on Thursday. A blurry screengrab of what is believed to have been the database, was released online as proof.

The hackers have claimed that the database will be searchable by Snapchat ID of the person who sent the photos. It was hosted on viralpop.com, Business Insider reported, and the site has now been suspended and taken down, but thousands have reportedly downloaded the images in the mean time.

Snapchat allows users to send photos that delete after a certain amount of time designated by the sender, on their phone or mobile device. But some apps or websites allow the person who receives the photos to save the image for a repeated viewing. It has been reported here that SnapSaved.com was hacked, while other news outlets have suggested it may have been the SnapSave app.

“We can confirm that Snapchat’s servers were never breached and were not the source of these leaks,” Snapchat said in a statement reported on Gigaom.

“Snapchatters were victimised by their use of third-party apps to send and receive Snaps, a practice that we expressly prohibit in our Terms of Use precisely because they compromise our users’ security. We vigilantly monitor the App Store and Google Play for illegal third-party apps and have succeeded in getting many of these removed.”

Snapchat has more than 30m users who collectively send around 700m photos and videos a day.

We can confirm that Snapchatâ??s servers were never breached and were not the source of these leaks. — Snapchat (@Snapchat) October 10, 2014