NEW DELHI (AFP) - Crucial talks between the Indian government and its left-wing allies on a controversial nuclear energy pact with the United States were postponed Wednesday with both sides still at odds over the deal. Neither side gave any reason for the delay but a source close to the talks, who asked not to be named, said left-wing parties were refusing to respond to a government appeal for the deal to go through. The official said the discussions would now be held on June 25. The government argues that the pact, which would give India access to the worldwide nuclear energy trade even though it has not signed global non-proliferation pacts, is crucial for the country's energy security. But opponents of the deal - the Communists who prop up the Congress government in parliament and the opposition Hindu nationalists say the pact brings traditionally non-aligned New Delhi too close to Washington and could compromise India's military programme. The latest extension of the long-running spat is another blow to the deal, which was concluded in 2006. The pact has to be passed by the US Congress ahead of November polls in the United States. Key US lawmakers have been stepping up pressure on New Delhi to clear the deal so that it can get final approval by the US Congress, where it currently enjoys bipartisan support. "The deal is virtually dead but the concerned sides need to keep talking about it," said political analyst and author Rasheed Kidwai. "After all, the (Indian) Prime Minister's (Manmohan Singh's) personal prestige is at stake," he added. Besides clearance from the US Congress, the pact needs approval from the International Atomic Energy Agency to place India's civilian nuclear reactors under UN safeguards. Reports Wednesday said Indian Foreign Minister Pranab Mukherjee met senior Communist and Marxist leaders earlier this week for their approval to sign a pact with the IAEA - a move staunchly opposed by both groups. India also requires the green light from the 45-nation Nuclear Suppliers Group, which regulates global civilian nuclear trade, before it can begin buying nuclear reactors and fuel.