The hack was reported by several media yesterday and confirmed by NiceHash in a post on Facebook, but the company did not disclose the amount of bitcoin that were stolen.

"Our payment system was compromised and the contents of the NiceHash Bitcoin wallet have been stolen. We are working to verify the precise number of BTC taken," the company said.

NiceHash is a popular pool for the mining of the cryptocurrency bitcoin used by many people who generate, or mine, bitcoin.

Various estimates suggest its holding of bitcoin was worth up to US$ 64m, all of which is now gone.

The company confirmed the hack after its site was down for hours for what NiceHash initially said was maintenance.

Many bitcoin exchanges have been the victims of hacks in recent years, but this is reportedly one of the biggest such cases.