The Communists have opposed the deal
Communist allies of India's government have given it a deadline to say if it will proceed with the civilian nuclear deal with the US.
The Communists, who say they will withdraw support from the government if it goes ahead with the deal, want the government to inform them by Monday.
The Communists have 59 members in the lower house of the parliament.
They say the deal would give the US undue influence over India's foreign and nuclear policy.
India is under pressure from Washington to sign the deal before the US presidential elections in November.
The government is holding talks with another party to bolster its support.
Reports suggest that the Congress party-led coalition would go ahead with the deal.
India's foreign secretary Shiv Shankar Menon has said that Indian Prime Minister Manmohan Singh will meet US President George W Bush on the sidelines of the G8 summit in Tokyo on Wednesday.
Correspondents say Mr Singh is expected to tell Mr Bush that India would be going ahead with the deal despite the opposition from its allies.
The deal now needs to be approved by the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), as well as by the 45-nation Nuclear Suppliers Group, which regulates global civilian nuclear trade.
Then it has to be presented to the US Congress for final approval.
The Communist allies met on Friday to discuss the "modalities" of withdrawing support to the government.
"We wish to know definitely whether the government is proceeding to seek the approval of the safeguards agreement by the board of governors of the IAEA," said Prakash Karat, head of the Communist Party of India (Marxist), reading out a letter addressed to the government.
"Please let us know the position by July 7, 2008."
The Congress says it will not be bound by the demand. "Sovereign governments or political parties cannot be subjected to deadlines," said Congress spokesperson Abhishek Singhvi
Another Communist leader AB Bardhan said there was no confusion about the Communists withdrawing support if the government went ahead with the deal.
"Only modalities like timing and writing to the president will have to be decided," he said.
Analysts say the decision to withdraw support could come in the next few days, plunging India into a fresh round of political uncertainty.
NUCLEAR POWER IN INDIA
India has 14 reactors in commercial operation and nine under construction
Nuclear power supplies about 3% of India's electricity
By 2050, nuclear power is expected to provide 25% of the country's electricity
India has limited coal and uranium reserves
Its huge thorium reserves - about 25% of the world's total - are expected to fuel its nuclear power programme long-term
Source: Uranium Information Center
Separately, the Congress party is holding talks with the regional Samajwadi Party on Friday to secure its support for the deal to compensate for the loss of its Communist allies.
If the Communists withdraw support, the ruling coalition would be reduced to 226 members in the 543-member parliament, a good 46 seats behind the majority mark of 272.
The Samajwadi Party, which has been a traditional political foe of the Congress, has hinted that its 39 MPs could end up supporting the ruling coalition on the nuclear deal issue.
If that happens, the ruling coalition will mop up the support of 265 MPs - only seven short of a majority.
Analysts say that securing the support another seven members will not be a problem as there are a number of "fence-sitters" from other smaller parties.
'Time running out'
Such an eventuality would help avoid early elections even after the Communists pull the plug on the government, they say.
Meanwhile, a delegation of US legislators visiting India have said that "time is running out for the deal".
"We are hopeful that the processes will move quickly," delegation leader Gary Ackerman said.
Reports say that Indian Prime Minister Manmohan Singh, who is believed to be the architect of the controversial deal, wants to proceed with it before the G8 summit beginning on Monday in Japan.
Under the terms of the controversial deal, India would get access to US civilian nuclear technology and fuel.
In return, Delhi would open its civilian nuclear facilities to inspection - but its nuclear weapons sites would remain off-limits.
US President George W Bush finalised the nuclear agreement with Prime Minister Manmohan Singh in July 2005.
It overturned three decades of US policy by allowing the sale of nuclear technology and fuel to India.
With President Bush's second and final term in office drawing to a close and presidential elections set for November, the Bush administration is growing increasingly keen to wrap up the deal.
Many analysts and some within the Bush administration believe a failure to conclude the agreement could create a setback for the current momentum in US-India relations.