Deputy Secretary of State Robert Zoellick, the department's No. 2 official, is resigning, Secretary of State Condoleezza Rice announced Monday.

Rice praised Zoellick's "tireless work ethic" and said he had served as her "alter ego" in the department.

"Our nation is stronger and safer because of your work," Rice said at a State Department announcement.

"I appreciate your confidence and friendship," Zoellick replied.

Zoellick, who served six years in the Bush administration, said he would join the Wall Street investment house Goldman Sachs Group Inc.

In his resignation letter, dated June 15, Zoellick, 52, did not say why he was leaving. A former U.S. trade representative, Zoellick reportedly wanted to be promoted to treasury secretary to replace departing secretary John Snow, but President Bush nominated Goldman Sachs executive Henry Paulson instead.

"I'm pleased to have built a first-rate team, offer counsel on a number of initiatives and backup the secretary," Zoellick said at the news briefing. "I've accomplished what I set out to do, and it's time to step down."

Zoellick is expected to leave the State Department in July, officials said.

He has been the administration's leading diplomat handling the volatile situation in Sudan's western Darfur region, where at least 180,000 have died since villagers began an uprising in 2003.

Zoellick has also been the State Department's chief diplomat dealing with China.

"Deputy Zoellick has been one of Secretary Rice's closest and most valued advisers on every single international issue," Rice senior adviser Jim Wilkinson said Monday, adding that Rice accepted the resignation reluctantly.

Zoellick told President Bush of his decision in recent weeks, officials said.