Nicholas Burns, a leading American policymaker on Iran and India, has decided to retire as the No 3 US diplomat for personal reasons, Secretary of State Condoleezza Rice said.

The White House quickly proposed US Ambassador to Russia William Burns to replace him as undersecretary of state for political affairs, a coveted assignment typically filled by a career diplomat. The appointment must be confirmed by the US Senate.

The two men are not related.

US officials said the State Department was expected to put forward John Beyrle, the US ambassador to Bulgaria, to head the US Embassy in Moscow but that the White House had made no decision.

"This is a very bittersweet time for us because Nick Burns had decided that it is time for him to retire," Rice told reporters as she announced his departure. "He has decided that it's the right moment to go back to family concerns."    

Burns, a former US ambassador to Nato and Greece as well as a former State Department spokesman, has played a major role in negotiating the two UN Security Council resolutions imposing sanctions against Iran over its nuclear program.

The foreign ministers of Germany and the five permanent members of the Security Council -- Britain, China, France, Russia and the United States -- will meet in Berlin on Tuesday to discuss a third sanctions resolution.

"He's going to continue to sprint until he leaves in March," Rice said of Burns, 51. "Next week we will go together to Berlin to have a little meeting on the way forward on Iran -- the P5+1. And so we have a lot of work to do."    

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 has also been the lead negotiator on a proposed deal to give India access to US nuclear equipment and fuel for the first time in 30 years even though it has tested nuclear weapons and refused to join nonproliferation agreements.

Rice said Burns would continue to work on the US-India nuclear cooperation agreement after he retired. The pact must clear several hurdles, including final approval by the US Congress, before taking effect.

The Bush administration sees the deal as the centerpiece of a new, strategic relationship between Washington and New Delhi and argues it will help India meet its soaring energy needs and provide business opportunities for US companies.

The agreement is controversial in India, where it is opposed by the communist allies of the Congress Party-led government, who believe it would infringe on Indian sovereignty. It has also been criticized by Western nonproliferation experts who fear it will undercut efforts to stop the spread of nuclear arms. - Reuters