“To minimize the damages to our customers, we will allow them to withdraw about 75 percent of their balance,” the statement said, adding the exchange was “very sorry” for the shutdown.

The remaining crypto assets were unharmed because they were stored in a “cold wallet,” a way of storing cryptocurrencies offline, YouBit said.

Police said they began investigating the case, and added they are planning to look into the possible involvement of North Korean hackers once the case is confirmed as an actual attack.

YouBit, formerly known as “Yapizon,” had been hit by a hack in April, when hackers stole 5.5 billion won (US$5.06 million) worth of cryptocurrency, according to local media reports.

Officials at YouBit were not immediately available for comment.

South Korea is one of the world’s busiest trading exchanges for cryptocurrencies. (Yonhap)