India and the US have achieved a breakthrough in talks over civilian nuclear deal with the two sides reaching an understanding on an agreement that will operationalise the deal but put off a formal announcement till completion of the political process.

After four days of intense official level negotiations the two sides reached an understanding on a common text of 123 Agreement.

"We have basically finalised the text but the document cannot be divulged," a senior Indian official told PTI.

The official said the document cannot be made public as it has to be approved by the Indian Cabinet.

Foreign Secretary Shivshankar Menon and US Undersecretary of State Nicolas Burns held tough discussion over last four days to resolve differences on issues like reprocessing right for India and fate of the deal if New Delhi were to conduct a nuclear test in future.

National Security Advisor M K Narayanan also met with Vice President Dick Cheney, Secretary of State Condolezza Rice, Defence Secretary Robert Gates and US National Security Advisor Stephen P Hadley.

Senior officials said that if a formal announcement has not been made, it is only within 'standard procedure,' in that the whole exercise will have to move away from diplomats and civil servants to the higher political levels.

"Much or all of the work has already been completed," a source familiar with the talks said, stressing that it has gone even beyond the 'crossing of the Ts and dotting the Is."

Sources say that an announcement, on India and the United States coming to an agreement on the 123 Accord, will be made simultaneously in New Delhi and Washington, probably during the possible visit of the Secretary of State, Condoleezza Rice sometime in August or September.

As the Bush administration also will have to take the senior law makers of the US Congress into confidence, just as in the manner it was done prior to the passage of the Hyde Act in December 2006.

Earlier, a joint communique was issued at the end of the final round of talks that said the discussions were 'constructive and positive' and both the Under Secretary of State for Political Affairs Nicholas Burns and the Indian Foreign Secretary Menon are pleased with the substantial progress made on the outstanding issues in the 123 agreement.

The two sides will refer the issue to their governments for final review.

"Both the United States and India look forward to the completion of these remaining steps and to the conclusion of this historic Initiative," the communique said.

The Deputy Spokesman Tom Casey, during a briefing at the States Department stressed that a lack of an announcement on the agreement of 123 negotiations will not mean that India and the US will not be ultimately be able to have a deal or move forward on the issue.

The spokesman when asked why it was that the US is not prepared to discuss publicly the issues that are at stake in the ongoing talks on the civilian nuclear accord.

"We sure will. And just like any other diplomatic negotiation, when an agreement is concluded, that will be a public document. It will have full review, I am sure, by appropriate committees and members of Congress, as well as by you guys and other interested members of the public," Casey said.

Meanwhile, members of the Indian delegation have left Washington after the four days of a hectic negotiating session.

Indian Foreign Secretary Shivshankar Menon, Chairman of the Atomic Energy Commission Anil Kakodkar and the Joint Secretary Americas in the External Ministry Gaitri Kumar have left for India while the leader of the Indian delegation National Security Advisor, M K Narayanan, will be leaving the US capital Saturday morning.