The bank has not confirmed reports that $133mn (€118mn) was stolen, but says the attack is under control and customers will suffer no financial loss.
Crédit Mutuel Arkéa said on Wednesday that a cyberattack had allowed fraudulent transactions, but insisted that none of its customers would suffer any financial loss.
The French bank, headquartered in Brittany, in western France, said its teams had detected the attack and that the incident was now under control.
The bank’s management said it had taken "the decision to temporarily suspend certain transactions in order to secure operations and prevent any new fraudulent movements".
According to the regional newspaper Le Télégramme, $133mn (€118mn) was stolen. The bank has not confirmed that figure, but it told another French newspaper, Le Parisien, that some funds had been frozen and the amount involved was still being assessed.
The fraud was first made public by ZachXBT, an independent investigator who specialises in tracking cryptocurrency transactions.
According to AFP, ZachXBT said the stolen funds had been routed through multiple accounts on cryptocurrency exchanges.
The bank said suspended transactions would resume shortly, once final security checks and measures had been completed. It did not explain how the attack was carried out or identify who might be responsible.