It said it was treating allegations levelled against Mr Adesina with the seriousness they deserved.

The US had rejected the findings of a probe made into allegations of nepotism - the bank’s ethics committee had exonerated Mr Adesina.

Niale Kaba, the bank's board chairperson, said there was no governance or constitutional crisis at the AfDB.

Mr Adesina is accused by a group of whistle-blowers of appointing relatives to strategic positions at the bank as well as giving contracts to acquaintances. He denies any wrongdoing.

The US holds a 6.5% stake in the bank, the second-largest shareholding after Nigeria.

Denmark, Norway, Sweden and Finland - who are also shareholders - have also written to the Abidjan-based lender backing calls for an independent probe.