Mizuho Bank, one of Japan's largest lenders, has became ensnared in a North American legal fallout from Mt Gox, once the world's biggest bitcoin exchange, which collapsed last month after losing nearly half a billion dollars worth of customers' digital currency.

Lawsuits in the US and Canada represent a new legal front in the battle over Mt Gox, which claims hackers stole huge amounts of its own and its customers' assets.

Mizuho, the core unit of Mizuho Financial Group Inc, Japan's second-biggest "megabank" by assets, was added as a defendant on Friday to an existing US lawsuit against Mt Gox for allegedly aiding in a fraud by providing services to the exchange.

Also on Friday, Mizuho was named in a class-action lawsuit in Canada against Mt Gox, alleging a security breach at Mt Gox resulted in "the pilfering of millions of dollars' worth of its users' bitcoins."

A Mizuho spokeswoman in Tokyo declined comment yesterday on the lawsuits.

Tokyo-based Mt Gox closed its virtual doors on February 25 and three days later filed for Chapter 11-style bankruptcy protection with a Japanese court.

The company said it had lost all 750,000 customer bitcoins it was holding, as well as 100,000 of its own and 2.8 billion yen (US$27.6 million) in cash. That marks US$567 million of vanished assets at current market prices, as well as about 7 percent of the bitcoins in circulation.