Mr Zoellick is an internationalist and free-markets enthusiast

Describing Mr Zoellick as her "alter ego", Ms Rice praised his "tireless work ethic" and said his efforts had made the US "stronger and safer".

Mr Zoellick, 52, has spent six years in the Bush administration.

He has led talks with Sudan over the violence in Darfur region as well as in Washington's dealings with China.

"I have accomplished what I set out to do and it is time for me to step down," Mr Zoellick said.

Mr Zoellick was a former US trade representative before becoming Ms Rice's deputy in early 2005.

He had been tipped as a candidate to take over as treasury secretary.

However, President George W Bush last month nominated a former Goldman Sachs executive, Henry Paulson, to take over the post from the outgoing John Snow.

Mr Zoellick will leave the government next month.

He has been described an enthusiastic advocate of free trade and an internationalist with a broad range of foreign contacts.

As US trade representative from 2001-05, he completed negotiations to bring China and Taiwan into the World Trade Organization.

Mr Zoellick, a Harvard-trained lawyer, was a senior international adviser to Goldman Sachs in the 1990s.