A statement on Crytek.com said: "We recently became aware of suspicious activity relating to some of Crytek's websites, and acted quickly to take those websites offline for security reasons. We thank you for your patience, and expect to have these sites fully operational soon."
Crytek.com, Mycryengine.com, Crydev.net, and MyCrysis.com all currently display the message, while the developer's other sites, GFACE.com, Crysis.com, and Warface.com are unaffected.
According to the report, users will be prompted to change their passwords once normal service is restored.
Game publishers and developers have become attractive targets for online attackers in recent years, with other high-profile intrusions including those against World of Tanks creator Wargaming.net, Blizzard Entertainment, Ubisoft, and Sony's PlayStation Network.
A spokesperson for EA, Crysis 3's publisher, was unavailable for comment at the time of publication.