Rice is expected to recommend US Ambassador to Russia William Burns to replace him as under secretary of state for political affairs, said a US official, who asked not to be named.

"This is a very bittersweet time for us because Nick Burns had decided that it is time for him to retire," Rice said. "He has decided that its the right moment to go back to family concerns."

Nicholas Burns, a former US Ambassador to NATO and State Department spokesman, has been a key player in negotiating the two UN Security Council resolutions imposing sanctions against Iran over its nuclear program.

The United States believes Iran aims to acquire nuclear weapons and the UN sanctions are designed to force Tehran to suspend its uranium enrichment activities, which can produce fuel for nuclear weapons or nuclear power plants. Iran says its program is for power generation.

Burns, 51, has also been the lead negotiator on a US-India civil nuclear deal that would give India access to US nuclear fuel and equipment for the first time in three decades.