US Undersecretary of State for Political Affairs Nicholas Burns, the number three US diplomat, is expected to step down for personal reasons in an announcement to be made on Friday, a US official said.
US Secretary of State Condoleezza Rice, who plans to make the announcement at 9:45 am EST (1445 GMT), is expected to recommend US ambassador to Russia William Burns as his replacement, said the official, who asked not to be named.
Nicholas Burns, a former US ambassador to NATO and State Department spokesman, has been a key player in negotiating the US-India civil nuclear deal that would give India access to US nuclear fuel and equipment for the first time in three decades.
The US official said Burns, whose is not expected to formally leave the department for a few months, is likely to keep a role for a few months after that in shepherding through the nuclear deal, which must clear several hurdles including final approval by the US Congress before it can take effect.
The US embassy in Moscow declined to comment on the possible move of its ambassador to Washington.
As ambassador to Moscow, William Burns has helped steer US-Russia relations through their most testing period since the end of the Cold War.
Despite often-hostile public statements from Russian officials and sharp disagreements between Washington and Moscow on missile defense, Kosovo and Iraq, Burns has worked quietly, shunning public attention, to calm the atmosphere and keep the lines of communication open between Washington and Moscow.
William Burns has wide experience, having served as the top US diplomat to the Middle East early in the current Bush administration and as US ambassador to Jordan.