Tokyo, Sep 20 (IANS): A Japanese cryptocurrency operator revealed on Thursday that hackers had accessed its systems last week and stole about 6 billion yen ($60 million).

Tech Bureau Corp said in a statement that the unauthorised breach from abroad took place for two hours on September 14 but it was only detected on Monday and the damage was not discovered until Tuesday, reports Efe news.

Sixty-seven per cent of what was stolen were assets of the firm's clients, while the rest were funds of a cryptocurrency exchange company integrated with Tech Bureau Corp, Zaif.

The theft affected three cryptocurrencies, but 89 per cent was in bitcoins (BTC) and the rest in the virtual currencies MonaCoin (MONA) and Bitcoin Cash (BCH), in amounts not yet accurately calculated, since the investigation is still underway.

The company said that the server is being thoroughly reviewed to prevent further damage, adding that it has requested financial assistance from one of its subsidiaries to cover the damage caused.

The damage suffered by this firm is less than a similar incident in January, when the Japanese operator Coincheck said its system was hacked and some $534 million were stolen.

These incidents forced the Japanese authorities to tighten controls on the cryptocurrency markets.