Bithumb said on Twitter it was temporarily suspending deposit and withdrawal services, following the security breach.
The Seoul-based operator said on Twitter that the stolen coins would be covered from Bithumb's own reserves and that all users' assets are being transferred to safer "cold wallet."
Tweet 1: [Notice for the temporary suspension of the deposits] Due to the increasing safety issues, we are changing our wallet system. Please do not deposit until we notify. *All deposits are not deposited into your wallet until all changes are completed.
Tweet 2: [Notice for the suspension of all deposit and withdrawal service] We checked that some of cryptocurrencies valued about $30,000,000 was stolen. Those stolen cryptocurrencies will be covered from Bithumb and all of assets are being transferring to cold wallet.
Tweet 3: *All deposit and withdrawal service will be stopped to make sure the security. We will keep notice you of the restart of the service. We apologize for your inconvenience and thanks for your understanding.
The exchange did not say when it would restart its deposit and withdrawal services.
Bitcoin fell from about $6,718.35 to as low as $6,561.79, following the tweets from Bithumb, according to CoinDesk's bitcoin price index. The price then started recovering and at 10:03 a.m. HK/SIN, bitcoin traded at $6,614.46.
Such a move is relatively staid for the regularly volatile asset.
CoinDesk data also showed that the price of ethereum fell from levels near $535 to as low as $521.07 before recovering slightly.
South Korea is a major cryptocurrency trading center and Bithumb is one of the busiest virtual coin exchanges.
Earlier this month, a relatively small South Korean exchange, Coinrail, tweeted that it had been hacked, and noted that lesser-known cryptocurrencies such as Pundi X were among those affected.
Some observers suggested that news affected bitcoin and sent the price on the Coindesk index to its lowest in two months. Still, given the relative size of Coinrail, that claim proved inconclusive.
That said, the decline also coincided with a broader crypto sell-off that followed a Wall Street Journal report that said U.S. regulators were investigating potential price manipulation at four major cryptocurrency exchanges.
Bitcoi, and ethereum then received a boost after a U.S. Securities and Exchange Commission official said that neither of those crypto assets are securities . At the same time, the SEC also said that many, but not all, initial coin offerings could be securities and would come under the regulatory control of the SEC and relevant securities laws.
— CNBC's Evelyn Cheng and Reuters contributed to this report.