KuCoin confirmed the security incident saying that it detected some large withdrawals since Saturday.
It found that part of Bitcoin, ERC-20 and other tokens in KuCoin's hot wallets were transferred out of the exchange.
While hot wallets refer to Internet-connected cryptocurrency management apps, offline wallets for storing cryptocurrencies are called cold wallets.
"The assets in our cold wallets are safe and unharmed, and hot wallets have been re-deployed," KuCoin said in a statement on Saturday.
"Please rest assured that if any user fund is affected by this incident, it will be covered completely by KuCoin and our insurance fund," it added.
According to a report in ZDNet, the loss is estimated at not less than $150 million, based on an Etherium address where users monitored some of the emptied funds.
However, KuCoin Global CEO Johnny Lyu said that the company was still evaluating the total value of the affected digital assets.
"The funds affected contain a small part of our total assets holdings," he said.
Answering a question on the reason behind the asset outflow, Lyu said that it was "due to the leakage of the private key of KuCoin hot wallets.a KuCoin said that a thorough security review of the incident will be conducted and the deposit and withdrawal service will be suspended during the period.
Lyu said that deposit and withdrawal service should be back within a week.