Blizzard has issued a World of Warcraft security warning following a recent increase in unauthorised account logins via its website and the mobile armoury app.

The studio said today: "We're in the process of notifying any account holders who were not using an authenticator and whose account showed signs of unauthorized access (e.g., logging in from an unusual IP address). If you are among this group, you will receive an email describing how to reset your account.

"As a result of these activities, access to the World of Warcraft auction house via the mobile app has been taken off-line temporarily. Upon request, our customer support team will restore in-game items and gold for any accounts impacted."

While Blizzard added that "no means of account security is guaranteed", it urged users to take precautions such as periodically changing their passwords.

World of Warcraft lost approximately 1.3 million subscribers in the first quarter of calendar year 2013, according to Activision Blizzard.

Activision CEO Bobby Kotick said further quarterly declines "are likely" as the year progresses, raising "concerns" as the firm heads towards the competitive holiday period. He said Blizzard intends to release new content more frequently in a bid to keep players engaged and make it easier for lapsed users to return to the game.

Blizzard confirmed last month that its unannounced MMO, codenamed Titan, is undergoing "large design and technology changes". According to reports, Titan is now not expected to be released until 2016.