Mizuho Bank has become ensnared in 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 United States and Canada represent a new legal front — and a deep-pocketed defendant — in the battle over Tokyo-based Mt. Gox, which claims hackers stole huge amounts of its assets and those of its customers.

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