India and the United States will hold another round of talks to narrow down differences on a proposed agreement to operationalise the civil nuclear deal when Foreign Secretary Shiv Shankar Menon undertakes a visit to Washington from Sunday.

Menon will hold talks with Under Secretary Nicholas Burns on the 123 agreement amid "frustration" in the US at the slow pace of negotiations and India's insistence on right to reprocess spent nuclear fuel and perennial cooperation even if it were to conduct an atomic test.

Significantly, the meeting between Menon and Burns on Tuesday will take place 10 days after senior officials from the two sides met 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 civil 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.