Japanese media said on Friday that a cryptocurrency exchange has lost 58 billion yen (530 million US dollars) because of hacking.
The Coincheck exchange said on its website that it had halted sales and withdrawals of a cryptocurrency called NEM.
It later added that it had restricted dealings in most other cryptocurrencies too.
Coincheck President Koichiro Wada apologised at a news conference and said the company may seek financial assistance.
The Kyodo News service said the Coincheck exchange detected the unauthorised access to its system early on Friday.
The reported loss tops the 48 billion yen (441.6 million US dollars) that the Japan-based Mt. Gox Bitcoin exchange lost in 2014.
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