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Hackers have massacred all the virtual characters in some of online adventure game World of Warcraft's major cities.

They appear to have created a character able to destroy those of other players and those controlled by the game.

Game creator Blizzard Activision has appealed for information, but added: "This exploit has already been hot-fixed, so it should not be repeatable."

More than 10 million subscribers play World of Warcraft (WoW), almost half are believed to be in China.

Numbers had dipped from a peak of 12 million in 2010 to 9.1 million at the beginning of 2012, but have swelled since the release at the end of September of an expansion called the Mists of Pandaria.

Writing in an online forum, Blizzard said it was taking the action "very seriously".

"Earlier today, certain realms were affected by an in-game exploit, resulting in the deaths of player characters and non-player characters in some of the major cities," wrote a representative going by the name of Nethaera, on Sunday.

Olivia Grace, contributing editor of website , said: "It was a significant hack.

"They discovered a method to roll a level-one [beginner] character, which ran to the major cities.

"We don't know exactly what they did, but somehow they were able to kill every single player's character in that city and every single computer controlled character - and they were doing this repeatedly."

Ms Grace said she believed the same hackers had attacked WoW before but not to this extent, with characters being destroyed again after "reincarnation" as is normal within the confines of the game.