It said in the attack, which had been been "contained", hackers gained access to Arkea's account to make transfers to other banks, not the accounts of clients, who did not suffer any losses.

Arkea added that the stolen funds were then converted into cryptocurrencies on specialized platforms, but said a portion had been blocked and other efforts were being taken to recover the funds.

Arkea said it had excluded the transfers being due to human errors and that it "was a sophisticated attack coming from the outside".

The institution did not give details on how the attack was carried out or who could be behind it.

In a statement to AFP, the Paris public prosecutor's office said it had opened an official investigation.

The investigation concerns an "attack on an automated data processing system by an organised group", it said.

The bank would not confirm the figure of $133 million in losses reported by the newspaper Le Telegramme, but confirmed the scale of the losses.

"The amount of funds concerned by the attack, which is still being evaluated, was not [...] such as to undermine the financial health and solidity of the group," it added.

The lender earned a net profit of 230 million euros in the first half of the year.

ZachXBT, an independent investigator who specialises in tracking funds on the blockchain – the system that underpins cryptocurrency transactions – revealed the fraud on his social media accounts earlier in the week.

The anonymous online sleuth said the stolen funds had been routed through multiple accounts opened on crypto exchange platforms.

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