Rovio, maker of hit multi-platform game Angry Birds, is being sued by U.S. firm Lodsys for patent violations, the Telegraph reports.

Lodsys claims that the Finnish company violated its patents, which cover the methods for purchasing new levels inside the Angry Birds game.

The European patent system requires that software inventions exhibit some physical effect from patentability, while the U.S. patent system is more lax, allowing for purely idea-based software patents.

According to the Telegraph, several European game developers have given up on launching their games in the U.S., and some are considering withdrawing games from the U.S. market due to patent claims by Lodsys.

Angry Birds is downloaded more than 1 million times per day. Originally a mobile game, it quickly spread onto many platforms, including browsers and consoles, and its success has prompted Rovio to even consider an IPO.