South Korea's daily multimillion dollar trade in virtual goods has been wiped out by an apparent hack attack.

Denial-of-service attacks appear to have taken down at least three key websites where the country's online gamers buy and sell 'gold' and other in-game items.

Official estimates from Korea's Ministry of Culture put the value of virtual cash trades at almost $900m in 2006.

Players of online role playing games commonly use real-world cash to purchase 'virtual gold' and other in-game currencies.

The players buy virtual cash to quickly gain an advantage over other players, or to avoid playing boring and repetitive parts of the game.

The three Korean sites where gold and items are traded - ItemMania, ItemBay and Item Play Forum - are still inaccessible.

A spokesperson for ItemMania told reporters that the site was "undergoing maintenance" to cope with an unexpected flood of visitors.

However, a smaller trading site, ItemRia, later posted a notice announcing that it had been the victim of a hacking attack. While the site is apparently now able to function, this notice is visible on ItemRia's home page.

Developing a high-level character in most online role playing games requires hundreds or even thousands of hours of play.

Busy professionals are therefore prepared to buy cash and in-game items to keep up with their peers.

Some particularly valued items change hands for hundreds of dollars, while high-level characters with all their assets can be sold for thousands.

In general, players purchase virtual items using a credit card at the virtual trading company's site, and then pick up the virtual currency or goods from the broker's agent inside the game.