Binance, the world’s largest cryptocurrency exchange, announced that it is investigating a security incident that may have led to the outflow of several billion dollars from the exchange’s wallets. Binance emphasized that users’ assets are safe and temporarily halted withdrawals on several networks.

Changpeng Zhao, the founder of Binance, said it appears that the exchange was hacked and that he expects users’ assets to be fully reimbursed.

Circle reportedly blocked about $800 million in Tether, and Tether Company also blocked around $1.56 billion in assets related to the attack.

Estimates of the amount taken differ. Observers in the cryptocurrency sector have reported an outflow of at least $2 billion, and some estimates put the probable figure at more than $5.7 billion.

The Wall Street Journal also reported, citing cybersecurity experts, that the value of the assets stolen may have exceeded $5.5 billion. The identity of the attackers has not yet been determined.