''That legal right has been preserved for the worst case,'' Mr Burns told a press conference in Washington, shown live at the United States Information Centre here.
''But we hope and trust that won't be necessay,'' he said.
The US official said India's offer to build a state-of-the-art reprocessing facility for spent fuel and commit it to IAEA safeguards was the turning point in the negotiations between the two countries on the civil nuclear cooperation.
He, however, added that the subsequent procedures for the reprocessing facility and the 123 agreement will be dealt with separately by the US Congress.
''The US will negotiate the subsequent procedures with India like it had done with Japan and Eurpopean countries.'' He said the Congress had the right to review it at every stage.
About the reprocessing facility, he said: ''We expect the Indian Government to cover the expenses.'' Mr Burns said he and US Secretary of State Condoleezza Rice were confident that the agreement would be cleared by the Congress.