The US faces an unprecedented challenge to its grip on the World Bank presidency, with emerging African economies nominating a candidate on Friday to set up the first contested bid for the top job at the global development lender.

Angola, Nigeria and South Africa endorsed the nomination of Nigerian Finance Minister Ngozi Okonjo-Iweala, a respected economist and diplomat, as a candidate to take over the bank when Robert Zoellick steps down in June.

'The endorsement is in line with the belief that the appointment of the leadership of the World Bank and its sister institution, the International Monetary Fund, should be merit-based, open and transparent,' the three said in a statement.

It was a rare example of unity among countries often at loggerheads as they strive for dominance on the continent.

Less than a day before the deadline to nominate, Washington has yet to announce its candidate - normally a shoo-in and still considered likely to be the appointee.

The US has held the presidency since the bank's inception after World War Two, and a European has always headed the IMF.

Brazil would like to nominate former Colombian finance minister Jose Antonio Ocampo, but it can't do so without Colombia's support, which now looks unlikely.

While Ocampo had agreed to stand and Brazil was willing to nominate him, Colombian Finance Minister Juan Carlos Echeverry said on Thursday his country was instead focusing on a bid for the presidency of the International Labour Organization.-Reuters