Known as Slingshot, the app's features include sharing photos and videos with friends and sending "reaction shots".
Like Snapchat, all images are deleted once sent and users can scribble or type over their photos.
Facebook has confirmed Slingshot's existence, but it is not known when the app will be officially released.
Reporters from The Verge and TechCrunch took screengrabs of the app's promotional material before it was removed by Facebook.
The images appear to reveal many of Slingshot's features, the more unusual of which include an unlocking mechanism, whereby photos received from friends must be unlocked by sending a photo back to the original sender.
It is thought that the back and forth "slinging" of images is why the app is called Slingshot.
"Earlier today, we accidentally released a version of Slingshot, a new app we're working on," confirmed Facebook in a statement.
The company did not reveal when the app would be made available, stating: "It'll be ready soon and we're excited for you to try it out."
Snapchat competitor?
In 2012 Facebook bought photo-sharing network Instagram for $1bn.
A year later, it was reported that Snapchat rejected a $3bn bid from Facebook, revealing the social media giant's apparent continued and serious interest in photo-messaging services.
Previously Facebook attempted and failed to create a successful image-messaging app called Poke, which was recently abandoned and had been described by some as a "blatant copycat app."
However, unlike Poke, Slingshot has a number of unique features not found in rivals such as Snapchat, which could make it a strong competitor.
Another similar app and potential rival is Taptalk, which is reportedly admired among some Facebook engineers.
Taptalk provides a comparatively minimalist and simplified approach to image messaging, allowing users to send personal pictures or videos by tapping or holding their friend's profile picture.
It has also been noted that Slingshot's icon is strikingly similar to Taptalk's.
Source: BBC News
