"The police are really good at understanding someone stole my credit card and ran up a lot of money. It's a lot harder to get them to buy into 'someone stole my magic sword,'" he told the Reuters news agency.

Mikko Hypponen a security specialist at security firm F-secure said that they have already dealt with hundreds of malicious programs that seek to steal players account details.

"It might sound far-fetched but this is a real problem," he told the BBC News website.

Black market

MMORPG allow players to take control of a character in a virtual world inhabited by a cast of thousands.

Popular games include Ultima online, Everquest and World of Warcraft. Players buy the game and then pay a monthly subscription to access the online world.

Gamers spend hours developing the skills of their characters and acquiring virtual objects to boost their strength and defeat their foes.

The power of a character is often directly linked to the amount of time a player is willing to devote to playing the game.

However, for players wanting to bypass this route a lucrative online market has developed buying and selling virtual objects such as potions and weapons.

This is often done through auction sites like eBay and other specialist fan sites.

Many of these transactions are against the terms and conditions of the games and must be carried out covertly.

Some estimates have put an $800m (£418m) price tag on the global market in game items and cash.

Last year, Sony launched the Sony Station Exchange for players of EverQuest to legally buy and sell in-game cash, magical items and characters.

Cash economy

This lucrative market has now become a focus for hackers and identity thieves.

Earlier this year, nearly a quarter of a million characters were created in the Korean online game Lineage using stolen identities.

Many of the characters were thought to have been put to work in "gaming farms", where gangs of criminals work thousands of characters to acquire objects to be sold for real world cash.

The Microsoft warning was made at the annual Gamesfest conference in Seattle.

"Those of you who are working on massively multiplayer online games, organized crime is already looking at you," Mr Weinstein told games developers at the conference.

He urged the games developers to be aware of the security risks.

Mr Hypponen of F-secure said the most common way for criminals to steal account details was by using trojans.

Trojans are an apparently harmless program that sit on a users computer collecting login and password information and sending it back to the hacker.

Often the trojans are disguised as programs that will give a character special powers such as invisibility. They may be distributed through games' chat rooms or by e-mail.

Once the hacker has the password details of a character they can login to the account and sell the characters weapons, potions and spells.

"For some users it's a tragic loss," he said.

Gamesfest runs in Seattle until 15 August.