NEW DELHI – India’s prime minister is raising fresh doubts about a landmark nuclear energy accord with the U.S., telling President Bush that his government is having “certain difficulties” finalizing the deal, which has faced mounting domestic opposition.

The pact would reverse three decades of American anti-proliferation policy by allowing the U.S. to send nuclear fuel and technology to India, which has been cut off from the global atomic trade by its refusal to sign nonproliferation treaties and its testing of nuclear weapons.

It has been billed as the cornerstone of a new partnership between India and the U.S. after decades of icy relations, and Washington is widely perceived to have made major concessions to make the pact acceptable to New Delhi.

But opposition in India has mounted in the months since the two sides finalized the deal’s technical aspects, with communist parties key to the survival of Prime Minister Manmohan Singh’s government arguing against closer ties with the United States.

The deal faces opposition in America, too. Critics there, including some in Congress, say providing U.S. fuel to India would free up India’s limited domestic supplies of nuclear material for use in atomic weapons, which they argue could spark a nuclear arms race in Asia.

In India, the feud had grown increasingly acrimonious in recent weeks, and there was widespread speculation about early elections until Friday, when Singh stepped back from the confrontation by saying it was “not the end of life” if the deal didn’t go through.

The doubts raised by that statement were further magnified when Singh told Bush on Monday that “certain difficulties have arisen with respect to the operationalisation” of the deal, according to a statement released yesterday by the prime minister’s office.