Amid growing 'frustration' in Washington over talks on a bilateral pact to operationalise the civilian nuclear deal, top officials of India and the United States met in Washington on Tuesday to iron out differences that have been nagging the talks for last few months.

Foreign Secretary Shivshankar Menon met US Under Secretary Nicholas Burns to sort out issues in the 123 agreement like right over reprocessing of spent fuel, nuclear testing and perennial supply of fuel for safeguarded Indian atomic facilities.

Both sides have expressed commitment to conclude the negotiations on the agreement as early as possible but the US has recently aired its 'frustration' at the slow pace of negotiations.

India's insistence on right to reprocess spent nuclear fuel and perennial cooperation even if it were to conduct an atomic test are among the differences that the two sides are seeking to wriggle out of.

Prior to Tuesday's meeting, the two officials held informal discussions on Monday evening.

Menon will also meet US Secretary of State Condoleeza Rice during his stay in Washington, indicating the seriousness with which the two sides are pursuing the matter.

Significantly, the meeting between Menon and Burns takes place ten days after the two sides held expert-level talks in the South African city of Cape Town.

Indian officials had said that 'some progress' was made during the discussions but some differences remained for which further parleys were required.

India, while noting its declared policy of unilateral moratorium on nuclear testing, refuses to accept it as legally binding by including a clause in the 123 agreement.

New Delhi insists that civilian nuclear cooperation should not be affected if India were to conduct a nuclear test and should be treated at par with other nuclear weapon countries in this regard.

While Washington may agree not to include the clause in the 123 agreement, perenniality of the nuclear cooperation becomes an issue as the US law provides for snapping of atomic ties if any country were to conduct a test.

India has also asserted that its right over reprocessing is not negotiable and the two sides are trying to narrow down differences on this matter as well.

Burns, who is the key interlocutor from the US side, will travel to India next month to push the negotiations further.

In the last several weeks, senior officials of the administration have expressed 'frustration' over the slow moving process and India's 'demands.'

"There is a strong sense of frustration in Washington, in the administration and in Congress, about the fact that the Indian side has progressed so slowly in this effort," Burns said in a newspaper interview in Washington recently. He said India needed to move quickly.      

He, however, added that the US did not 'question the goodwill of the Indian government' and said, "I believe we will overcome the problems we are encountering."      

In a separate interview, Burns said three rounds of talks with India have produced little.

New Delhi sees these statements as a negotiation tactic.