The African Development Bank on Thursday launched an independent inquiry into allegations of embezzlement and preferential treatment against its embattled president Akinwumi Adesina, who is seeking re-election in August, its board of governors said.

The investigation into Adesina, a former Nigerian minister for agriculture, follows calls for a probe by Washington, and must wrap up within a "maximum" of four weeks as the institution prepares to elect a new head, the board said in a statement.

Adesina—who was recently exonerated by an internal inquiry—is the first Nigerian to be in charge of the 56-year-old AfDB, one of the world's five major multilateral development banks.

On Tuesday, Nigerian president Muhammadu Buhari told Adesina that the country "will stand solidly behind" him in his bid to remain at the helm of the bank.

The bank chief repeated to Buhari his insistence the allegations against him were trumped up and that the US call for a new probe was against the bank's rules.