Bitcoin plunged after one of the largest exchanges halted trading because hackers stole about $65 million of the digital currency.

Bitcoin slumped 5.3 per cent against the US dollar on Wednesday morning in Tokyo, bringing its two-day drop to 13 per cent. Prices also sank 6.2 per cent on Monday, although it was not clear if that initial move was related to the hack.

Hong Kong-based exchange Bitfinex had said on Tuesday it halted trading, withdrawals and deposits after discovering the security breach. The exchange said it was still investigating details and cooperating with law enforcement, but acknowledged that some bitcoin have been stolen from its users.

Hackers took 119,756 bitcoin, or about $US65 million at current prices. Louie Douvis

"Yes - it is a large breach," Fred Ehrsam, co-founder of Coinbase, a crypto-currency wallet and trading platform, wrote in an e-mail. "Bitfinex is a large exchange, so it is a significant short-term event, although Bitcoin has shown its resiliency to these sorts of events in the past."

Bitfinex confirmed to Bloomberg News that the hackers took 119,756 bitcoin, or about $US65 million at current prices. More than $1.5 billion has been wiped off from bitcoin's market capitalisation this week, according to research from CoinDesk.