Premier Manmohan Singh, who had been pushing for the conclusion of the deal as his key foreign policy achievement, conveyed the message to US President George W. Bush during a phone conversation late Monday, the government said.

Singh "explained to President Bush that certain difficulties have arisen with respect to the operationalisation of the India-US civil nuclear cooperation agreement," a statement said.

In New Delhi, Foreign Minister Pranab Mukherjee also met the US ambassador to India, David Mulford, to explain the problems, a foreign ministry official told AFP.

The deal, finalised in August after two years of complex negotiations, had been championed as a the start of a new era of ties between the United States and one of Asia's fastest growing economies.

It is aimed at bringing energy-hungry India into the loop of global atomic commerce after a gap of three decades by allowing it to buy civilian nuclear technology despite possessing atomic weapons and not having signed the nuclear Non-Proliferation Treaty.

In exchange, India must put selected nuclear facilities under international safeguards, including inspections.

Singh and ruling Congress party chief Sonia Gandhi insist the deal is crucial for future growth, but have failed to win over India's Communists and other left-wing parties, who prop up the government in parliament.

The opponents are worried that traditionally non-aligned India is getting too close to Washington, and that the government may be compromising the future development of the country's nuclear weapons programme.

Left-wing parties have been threatening to withdraw their support for the government in parliament over the deal, a move that would force early elections.

"I think the government has accepted defeat," said Lalit Mansingh, a former Indian ambassador to Washington.

"I would presume the deal is dead as the timetable seems unrealistic," he added.

New Delhi must still sign a separate pact with the International Atomic Energy Agency (IAEA) and get the thumbs-up from the 45-member Nuclear Suppliers' Group, which controls global nuclear commerce.

This needs to be done before getting final approval from the US Congress, before elections there in late 2008.

C. Uday Bhaskar, former head of the Institute of Defence Studies and Analyses, agreed that "the deal has gone into a long pause mode."

According to a senior Indian Communist Party official, the government was forced to reconsider when its other legislative allies expressed their reluctance to face early elections.

On Friday, Singh admitted that "one has to take certain disappointments."

"For Congress allies, the nuclear deal is not an issue on which elections can be fought and won, though it would have improved India's image abroad," said political analyst Rasheed Kidwai.

"They are more interested in issues that the electorate are bothered about."

Unnamed US officials meanwhile told the Washington Post they were deeply disappointed with the decision, which they described as unexpected.

US Undersecretary of State Nicholas Burns -- who in early May reported "extensive progress" on the deal -- and other top US officials frantically worked over the weekend to try to revive the deal, according to the Post.

In late June, US Secretary of State Condoleezza Rice described the deal as "historic and pathbreaking," and said it could be clinched this year with enough commitment from both sides.

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