DMM Bitcoin reassured customers that their bitcoin deposits would be fully secured following a hack that led to an "unauthorized leak" of over $300 million worth of bitcoin, as disclosed in a blog post on Friday.

According to the post, the breach resulted in the theft of 4,502.9 BTC, valued at $305.1 million at current market rates. DMM Bitcoin pledged to guarantee customer bitcoin deposits by acquiring an equivalent amount of BTC with the assistance of its group companies, although no specific timeline for this procurement was provided.

The unauthorized transaction occurred at 1:26 p.m. Tokyo time, coinciding with a flagged transaction of 4,502 BTC noted by the Whale Alert account on X, formerly known as Twitter.

In response to the breach, DMM Bitcoin halted processing crypto withdrawals, spot market purchases, new leveraged trading positions, and is "screening" new account registrations. Withdrawals in yen may experience delays, as mentioned in the blog post.

Although the exchange did not specify a timeframe for resuming services, it indicated that a separate announcement would be made with further details.

As per a report from Nikkei, DMM Bitcoin had around 370,000 customer accounts by the end of 2023, according to its annual business report. If confirmed, the stolen amount would mark one of the largest crypto exchange attacks to date.

Notable incidents in Japan's crypto exchange history include Coincheck's theft of 58 billion yen (approximately $533 million) worth of crypto in 2018, which led to its acquisition by online brokerage firm Monax. The infamous collapse of Mt Gox in 2014, resulting in losses exceeding $400 million, initiated a prolonged process culminating in forthcoming fund distributions to creditors.