Political uncertainty has deepened in India with the communist allies of the government hardening their stance against the nuclear deal with the US.
The latest crisis was triggered by Mr Singh's comments on the deal
The communists say the deal, under which India gets access to civilian nuclear technology and fuel, gives the US leverage on India's foreign policy.
Other allies have backed the agreement amid fears that communists may withdraw support for the government.
This is one of the worst crises facing India's Congress-led government.
It has had an uneasy relationship with its communist allies ever since it came to power in 2004.
A flurry of meetings over the weekend failed to break the impasse, with the communists insisting on stalling the deal.
There were reports that the government has suggested setting up a committee to discuss the details of the deal to placate communist fears.
But a leader of the Communist Party of India (Marxists), the key communist allies, said there should be no "movement" on the deal before they agreed on any such committee.
'Loss of credibility'
"The government must first tell us whether it is prepared not to proceed with the implementation of the deal," Mr Prakash Karat said.
Essentially, the communists want the government to stall the upcoming meetings with the International Atomic Energy Agency (IAEA) and the 45-member Nuclear Suppliers Group (NSG) as a follow-up to the deal.
Analysts say stalling or calling off the deal may lead to the collapse of the Congress-led government, and isolation of India abroad.
The main opposition party, the Hindu nationalist Bharatiya Janata Party (BJP) party, has said they are ready for elections if the communists withdraw support to the government.
The communists insist that bringing down the government is not a "priority".
Communist leader Prakash Karat said their priority was to maintain "a secular government".
The Congress government has said it is confident of resolving its differences with the communists despite the difficult situation it finds itself in.
"A way out is possible," Foreign Minister Pranab Mukherjee told reporters.
The crisis was triggered last week by PM Manmohan Singh's refusal to back down from the deal and even challenging the communists to withdraw support for his government.
There have been some isolated protests against the nuclear deal
Mr Singh has described the agreement as "historic", saying it will open new doors to India across the world.
He said the deal would not in any way affect India's right to carry out nuclear tests in the future or inhibit the country's nuclear weapons programme.
Under the agreement, India is allowed to reprocess spent nuclear fuel - something that is seen as a major concession and opposed by some members of the US Congress as India has not signed the Nuclear Non-proliferation Treaty.
Last week, Australia decided to sell uranium to India, overturning the country's long-standing rule of not exporting to non-signatory nations.
Prime Minister John Howard said the decision was the best way to bring India into the nuclear mainstream, and would curb global warming by promoting nuclear power.