Binance, the world’s largest cryptocurrency exchange, is investigating a possible security problem after a series of large transfers from its platform. The reports were followed by the freezing of more than $2.3 billion by stablecoin issuers Tether and Circle, while bitcoin fell.

Binance founder Changpeng Zhao wrote on X that the activity appeared to be a hack. The exchange said users’ funds were safe. Those statements leave an important distinction unresolved: suspicious transfers have prompted an investigation, but the available information does not confirm a breach or establish whether users have suffered losses.

The amount frozen by Tether and Circle is not, on the evidence supplied, a confirmed measure of money stolen. No further details were provided about the transfers or the possible security problem.

Bitcoin traded at about $78,700 overnight, down roughly 3% over the preceding 24 hours, against the backdrop of the reports. The immediate uncertainty is the nature and extent of the suspected incident, despite Binance’s assurance about customer funds.