ABUJA - Nigeria's and Rwandan Finance Minister Donald Kaberuka face a run-off in July for the post of African Development Bank (AfDB) president after initial ballots ended in deadlock.

The chairman of the bank's board, Ngozi Okonjo-Iweala, said on Thursday a string of ballots in Abuja had failed to produce a clear winner between the two leading candidates under an electoral system requiring them to win a double mandate.

Six candidates were put forward to succeed Moroccan Omar Kabbaj, whose second term and final term ends soon.

To win, a candidate needed a majority of the total votes plus a majority of regional African votes. Regionals have 60 percent of the vote, while non-regionals have 40 percent.

"None of them won the outright requirement," Okonjo-Iweala said at the end of the bank's annual meeting in Abuja.

Okonjo-Iweala who doubles as the Nigeria's Finance Minister, said Ogunjobi, currently AfDB Vice-President, and Kaberuka would face another contest in the Tunisian capital of Tunis from July 21-22.

The African development finance institution established 40 years ago to help the continent's then newly independent countries to grow their economies, now has a capital base of around $32.5 billion.

The bank has two concessional windows, the African Development Fund set-up in 1976 by non-regional members and the Nigerian Trust Fund established in 1976 by the Nigerian government.

The ADB board comprises 51 African nations and 25 non-regional members- Western nations and donor agencies.