Undersecretary of State Nicholas Burns, the American pointsman for the landmark Indo-US civil nuclear deal, is resigning in March but will continue to handle the controversial deal which got bogged down in Indian politics.

The surprise decision by 51-year-old Burns, who negotiated the deal for more than two years with the then Indian Foreign Secretary Shyam Saran, was announced by Secretary of State Condoleezza Rice.

"He decided it was time for him to retire from the Foreign Service," Rice told reporters at the State Department on Friday adding, Burns will stay in his post until March.

And even after that, Burns will "continue to work on the Indian file," particularly on the nuclear deal which is yet to be implemented, Rice said.

"This is a very bittersweet time for us," Rice said referring to Burns' exit after a 25 year career in the American foreign service.

An American official, who did not want to be named, said Burns has been asked to stay on by Rice in the capacity of special envoy to deal with the unfinished civilian initiative with India.

Burns will be replaced by the current US Ambassador to Russia [Images] William Burns for the third-highest job of undersecretary of state for political affairs in the state department.

Personal reasons were cited for the exit of Burns, who is expected to take up a job in the private sector. He was widely viewed as an experienced and skillful diplomat and also dubbed by some as the most gifted foreign officer of his generation.

A career diplomat and a former spokesman of the state department, Burns was was a forceful administration voice for the deal both at the time of the passage of the Henry Hyde Act of 2006 as also while hammering out the crucial 123 Agreement with India last year.

In stressing the criticality of the civilian nuclear initiative between the United States and India, Burns was a consistent senior administration official calling to look at the larger aspects of the bilateral relationship.

"As we Americans consider our future role in the world, the rise of a democratic and increasingly powerful India represents a singularly positive opportunity to advance our global interests.

There is a tremendous strategic upside to our growing engagement with India. That is why building a close US-India partnership should be one of the United States' highest priorities for the future. It is a unique opportunity with real promise for the global balance of power," Burns recently wrote in Foreign Affairs.

Burns joined the foreign service in 1983 and worked at posts in the Middle East and Africa before taking over the Russia portfolio at the National Security Council in the waning years of the Cold War, during the administration of President George H W Bush.