The United States stepped up lobbying on Thursday for its pick to retain the top job at the World Bank as emerging markets pushed their view that the time has come for rich nations to share leadership of key institutions.

In a letter to World Bank board members, Treasury Secretary Timothy Geithner urged support for Korean-born Jim Yong Kim and flagged his experience from Asia to Africa to the Americas working on development projects.

But in New Delhi, a summit of the BRICs nations - Brazil, Russia, India, China and South Africa - called for leadership selection for the World Bank and International Monetary Fund through an open and merit-based process.

They said the World Bank should have a governance structure that reflects current economic and political reality, a reference to the five BRICs nations that account for nearly half the world's population and one-fifth of its output.

Since their founding after World War Two, the World Bank has been led by an American and the International Monetary Fund by a European - a convention that emerging markets find increasingly galling because their rising economic might is not reflected in opportunities at the top of key forums.

Two candidates from emerging-market countries, Nigerian Finance Minister Ngozi Okonjo-Iweala and former Colombian Finance Minister Jose Antonio Ocampo, have also been nominated to take over as World Bank president when Robert Zoellick steps down in June.

But the United States, with likely backing from Europe, is considered in top position to be able to install Kim at the bank's helm. Geithner stressed Kim's credentials as leader of Ivy League Dartmouth College and as an activist in promoting health care and development in poor countries.

Dr. Kim is ... a skilled manager and leader, demonstrating the entrepreneurial skill to grow Partners in Health into a global institution and the management know-how to successfully lead large programs and institutions at the World Health Organization, Geithner wrote.

A decision on a successor to Zoellick is expected by the time the World Bank and IMF hold semi-annual meetings in Washington April 20-22.

At the New Delhi conference, BRICs countries said the World Bank must change from an institution that essentially mediates North-South cooperation (between rich countries) to an institution that promotes equal partnership with all countries.

Geithner appeared to be trying to appease some of those concerns by underlining Kim's commitment to development issues.

He understands the increasingly important role of the bank in dealing with today's global challenges, including climate change, food security, and the vulnerability of fragile states, and will skillfully convene our countries to address these new and evolving issues, Geithner wrote to World Bank board members, all of whom have finance or development backgrounds.

(Reporting By Glenn Somerville; Editing by Padraic Cassidy)