ABN said it was regarded as a suspect in the investigation "due to its involvement in certain transactions", in which a third party had offset dividend withholding tax credits against corporate tax liabilities in the Netherlands.

An appeals court overturned an initial positive ruling for the unnamed third party last year, ABN said, and an appeal at the Supreme Court is pending.

The bank said it could not yet estimate the possible financial consequences of the investigation, as the timing of its completion and the outcome were still uncertain.

CEO Robert Swaak said the case was not related to the so-called "cum-ex" scandal, Germany's biggest post-war fraud involving a share-trading scheme that authorities say cost taxpayers billions of euros through bogus dividend tax rebates.

ABN on Wednesday said it had set aside 79 million euros ($92.5 million) for possible repayments demanded in relation to this case.

In April, the bank agreed to pay 480 million euros to Dutch prosecutors to settle a criminal investigation into its lax oversight of money laundering through its accounts.

One of three dominant banks in the Netherlands, ABN on Wednesday reported a second-quarter net profit of 393 million euros.

($1 = 0.8535 euros)

(Reporting by Bart Meijer; editing by Jason Neely)