The powerful head of India's ruling Congress party stood firm behind a controversial nuclear deal with the United States on Tuesday as fresh efforts were launched to convince communist allies who have rejected it.

Sonia Gandhi, the Italian-born chief of the party, called opposition to the deal sloganeering, a day after some allies and opposition parties shouted down the prime minister's statement defending it in parliament.

The unrelenting opposition by four communist parties, whose support is critical for the survival of Prime Minister Manmohan Singh's coalition, has sparked fears that it could destabilize the government nearly two years before national polls are due.

Our government has entered into this agreement after tough negotiations, Gandhi told Congress party MPs. The agreement fulfils all the assurances that the prime minister has given repeatedly in parliament.

The objectives of technological self-reliance and national sovereignty have been and will continue to be fully protected, she said of the deal which governments in New Delhi and Washington have hailed as historic.

We are a democracy and differences in views are inevitable but informed debate and discussion are the answer.

The nuclear deal aims to give India access to U.S. nuclear fuel and equipment for the first time in 30 years to help meet its soaring energy needs, even though it has stayed out of non-proliferation pacts and tested nuclear weapons.

First agreed in principle two years ago, the framework deal was approved by the U.S. Congress last December and the pact that governs nuclear trade between the two, called the 123 agreement, was finalized last month.

The 123 agreement has to get the backing of the U.S. Congress after India secures other international approvals.

Critics in both countries say their governments are making too many compromises in their eagerness to seal it. The pact is expected to face dissent in the U.S. Congress but would ultimately be approved, an influential senator said this week.

The communists, who have about 60 MPs in the 545-member lower house of parliament, say the deal compromises India's sovereignty and brings it under American strategic influence.

Gandhi's statement came as Singh and Foreign Minister Pranab Mukherjee held talks with Prakash Karat, general-secretary of the Communist Party of India (Marxist), the largest of the four left parties, about the deal.

Mr. Karat and the prime minister reiterated that efforts would be made to sort out the issues, Sanjaya Baru, Singh's spokesman, told reporters, adding that Karat said the issue would be placed before a party politburo meeting this week.

Analysts said opponents of the deal were letting their political ideologies overtake national interest.

If Marxists are not to lose any more ground in a country that's turning increasingly assertive and confident, they need to invent a politics that goes beyond old-fashioned whingeing, the Times of India said in an editorial on Tuesday.

(Additional reporting by Palash Kumar)