The African Development Bank says it has commissioned an independent review of a report which had cleared into its head, Akinwumi Adesina, of allegations of corruption.
The former Nigerian agriculture minister is seeking re-election in August.
But the United States has led calls for him to be investigated on suspicion of embezzlement, preferential treatment for fellow Nigerians in senior appointments, and the promotion of people suspected or convicted of fraud and corruption.
Mr Adesina denies any wrongdoing and was initially cleared of the allegations by an internal investigation.
The institution - one of the world's five major multilateral development banks - is the only African lender afforded the highest credit rating.
Correction: This post has been amended to clarify that the African Development Bank has commissioned a review, rather than an inquiry.