A group of communist parties that support the Congress party-led coalition asked Singh not to go ahead with the complex deal known as the 123 agreement because it requires several steps in tandem by both nations before coming into force.

"Our party has stated very clearly that the accord contains provisions that cannot be accepted by any country that loves its sovereignty," Prakash Karat, chief of the Communist Party of India (Marxist) told reporters.

The communist said they were unable to accept the pact after careful assessment of the text, released on Friday, in the "context of (a) burgeoning strategic alliance" with Washington.

The statement came ahead of India's monsoon session of parliament beginning Monday, in which the deal is also expected to run into criticism from the main opposition Bharatiya Janata Party.

Singh, who personally oversaw two years of talks that led to the historic pact, is expected to make a statement in parliament on August 13.

The terms of the accord, which seeks to bring India into the loop of global nuclear commerce after a gap of three decades, were also rejected by India's main opposition Hindu nationalists over the weekend.

The deal clinched in Washington last month allows India to buy civilian nuclear technology while possessing nuclear weapons, making it an exception under the Nuclear Non-Proliferation Treaty.

One of the trickiest issues -- whether India's unilateral decision to test nuclear weapons would end the deal -- appeared to have been sidestepped.

National Security Adviser M.K. Narayananan last month described the deal as "excellent" and India's atomic energy board chief, Anil Kakodkar, added that he was satisfied with it.

The deal must also be approved by the US Congress and other nations under the umbrella of the Nuclear Suppliers Group.

Within minutes of the Communists rejecting the deal, former Hindu nationalist foreign minister Yashwant Sinha, voiced his party's opposition, saying the accord would jeopardise India's nuclear weapons programme as well as restricting its foreign policy options.

"Such a long-term decision should be decided with a consensus," Sinha said. The deal, if passed by the US Congress, will be valid for 40 years.

Though the government rejected the Hindu nationalists' demand for a multi-party parliamentary committee to examine the deal, Sinha said his party "will raise this issue strongly in parliament and try to convince and compel the government to accept (it)."

Indian foreign minister Pranab Mukherjee said the government would talk to the Communists.

Politicial analyst Rasheed Kidwai said the Communists' objections to the deal put Singh and his government in a spot.

"The nuclear deal has been touted by the government as one of its greatest triumphs with Singh himself taking a lot of interest in this.

"A lot of hard work had gone into this and now when it is nearly done, the government's allies are opposing it," Kidwai said.

After the Hindu nationalists made it clear they will oppose the deal, the Communists will be compelled to support them against the government or risk being labelled inconsistent, he said.

"Such a move will make it untenable for the government to continue in office," he added.

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