Japan's Nintendo said Thursday it has filed a lawsuit accusing the developer of a video game jokingly dubbed "Pokemon with Guns" of patent infringement.

Nintendo and The Pokemon Company are suing Pocketpair, the Japan-based studio behind the "Palworld" game that became a viral hit when its prototype was released in January.

"Palworld" -- which sold more than five million copies in just three days -- features weapon-wielding player avatars and monsters that look similar to the creatures of Nintendo's wildly popular "Pokemon" franchise.

A Nintendo statement said the companies had "filed a patent infringement lawsuit in the Tokyo District Court against Pocketpair" on Wednesday.

"This lawsuit seeks an injunction against infringement and compensation for damages on the grounds that Palworld... infringes multiple patent rights," it said.

The statement did not specify how much compensation was being sought.

Nintendo added it would "take necessary actions against any infringement of its intellectual property rights" to protect what it has "worked hard to establish over the years".

"Palworld" is still in development but an "early access version" costs $29.99 for US customers on the online Steam platform.

"Fight, farm, build and work alongside mysterious creatures called 'Pals' in this completely new multiplayer, open world survival and crafting game!" its description states.

Initial reviews of the game were positive, although many pointed out the eerie resemblance of the "Pals" to Pokemon characters, with critics and users nicknaming it "Pokemon with guns".

"Most people, myself included, thought this game was going to be a meme," the account of game developer Pirate Software said in one review left on Steam.

"It's actually insanely detailed, extremely well optimised, and compelling as hell."

Pocketpair said in a statement Thursday that it will "begin the appropriate legal proceedings and investigations into the claims of patent infringement". 

"It is truly unfortunate that we will be forced to allocate significant time to matters unrelated to game development due to this lawsuit," it added. 

Pokemon has been a global hit since it was launched as a role-playing game in 1996 for Nintendo's Game Boy console. 

The franchise, whose slogan is "Gotta Catch 'Em All", also includes movies and a hugely popular animated TV show.