With the breakout success of battle royale shooter PlayerUnknown’s Battlegrounds, it was inevitable that other studios would start creating similar experiences. One of the most aggressive has been Epic Games, which announced a battle royale mode for its survival game Fortnite, which will be free to all players starting September 26th. Epic has even gone so far as to cite PUBG as an inspiration. “We love battle royale games like PUBG and thought Fortnite would make a great foundation for our own version,” Epic explained.

Now, PUBG developer Bluehole has issued a response to what it describes as “growing concerns” about the similarities between the two games. “We’ve had an ongoing relationship with Epic Games throughout PUBG’s development as they are the creators of [Unreal Engine 4], the engine we licensed for the game,” Chang Han Kim, Bluehole’s vice president, said in a statement. “After listening to the growing feedback from our community and reviewing the gameplay for ourselves, we are concerned that Fortnite may be replicating the experience for which PUBG is known.”

Bluehole says its concerns extend beyond just the basic, last-player-standing theme of both games, and also include similarities with Fortnite’s interface and structure. “We have also noticed that Epic Games references PUBG in the promotion of Fortnite to their community and in communications with the press,” Han Kim said. “This was never discussed with us and we don’t feel that it’s right.”

PUBG, which released as an early access title in March, has been a runaway success, selling more than 10 million copies to date. On Steam, the game recently broke the record for most concurrent players ever, topping 1.3 million. Its audience is set to grow as well with a Microsoft-published version of the game coming to Xbox One. Fortnite, meanwhile, debuted in July as a sandbox survival game, but this month Epic revealed plans for its new battle royale mode.

Bluehole doesn’t say what — if any — action it will be taking beyond just issuing a statement on the matter. “The PUBG community has and continues to provide evidence of the many similarities as we contemplate further action,” said Han Kim.