6.7 billion yen ($60 million) in cryptocurrency has been stolen by hackers from a Japanese digital exchange called Zaif.

Tech Bureau Corp, the Osaka based company that owns the exchange, said hackers gained access for a timeframe of over two hours on September 14 and stole money from Zaif’s ‘hot wallet’ where Bitcoin and other digital currencies are stored, Reuters reported.

Server problems were detected on September 17, with the company sounding the alarm to authorities the following day. The exchange was taken offline, and efforts have been underway to get it working again.

Hackers have stolen 6.7 billion yen ($60 million) in Bitcoin and two other cryptocurrencies from the Japanese digital exchange, Zaif, owned by startup Tech Bureau Corp, the company said on Thursday.

In a statement following the hack, Tech Bureau said that its Zaif exchange was hacked in a window of over two hours on September 14. They detected server problems on September 17, confirmed the hack and sounded the alarm to authorities the following day, Reuters reported.

Tech Bureau Corp said the perpetrators gained access to its 'hot wallet' where the digital coins are stored. The platform has been taken offline, but it said efforts were underway to get it working again.

Japan has been a leader in cryptocurrencies and has set up a system of licensing digital exchanges with the government to regulate the market and protect consumers. The system is designed to make Japan a world leader in the financial technology.

Bitcoin has been legal tender in Japan since April 2017, and some retailers in the country already accept the digital payments, The Washington Post reported. But Thursday’s hack and others before it show the technology is still having teething problems.

The digital exchange, Coincheck, which is based in Tokyo, reported that 58 billion yen ($547 million) in cryptocurrencies disappeared earlier in the year, in what was suspected to be another theft by hackers.

Coincheck had been applying for a government licence since 2012, but still didn’t have at the time it was hacked, sparking debate in the industry. Zaif was registered by the government last year.

On Thursday, after the hack, the company said it had accepted a 5 billion yen ($45 million) offer of investment from Tokyo based company Fisco, for a majority stake in Tech Bureau.

Bitcoin and Manacoin were among the cryptocurrencies taken in the September 14 breach. 2.2 billion yen ($20 million) of the stolen currency belonged to the company and the rest were owned by customers, Tech Corp said.