Japanese crypto exchange DMM Bitcoin said it had lost bitcoin worth around $300 million in what it described as "an unauthorised leak" from its digital wallet.

"We are still investigating details of the damages," it said in a statement after detecting the "unauthorised leak" at around 13:26 (0426 GMT) Friday.

"We have already taken measures to prevent the unauthorised leak, but we have also implemented restrictions on the use of some services to ensure additional safety," the exchange said.

DMM Bitcoin did not give details on the "leak" but global cryptocurrency analysis firm Chainalysis said it was one of the biggest crypto hacks.

"Today's hack of Japanese exchange DMM Bitcoin for $305M worth of bitcoin is the biggest hack since Dec 2022 and the 7th largest crypto hack ever," Chainalysis said in post on social media platform X, adding it had labelled the funds as "stolen".

DMM Bitcoin said the amount of the "leak" was about 48.2 billion yen ($300 million), which equals to 4,502.9 bitcoins.

All of its customers' "Bitcoin (BTC) deposits will be fully guaranteed" with support from its group companies, the company said.

Japan's Financial Services Agency has ordered the company to investigate the cause of the incident and protect customers from damages, while police have started probing the case, the business daily Nikkei reported.

Despite their growing popularity, cryptocurrencies have a history of scandal and sudden price collapses.

Thieves stole 850,000 bitcoins from Japanese exchange Mt. Gox in 2014. It was worth around $470 million at the time.

Another Japanese exchange, Coincheck, was hacked for nearly $500 million in 2018.